Epidemiology (Forensic Science)
Epidemiology, a very old science, is concerned with the risks associated with disease exposure, the identification and control of epidemics, and the monitoring of population rates of disease and exposure. In ancient Greece, Hippocrates first suggested that human disease may be related to external as well as personal environment. During the seventeenth century, the English statesman and philosopher Francis Bacon developed a set of inductive laws that formed the foundation of modern epidemiology. In 1662, John Graunt, a London haberdasher, collected mortality statistics and was the first to quantify patterns of disease in a population. In 1747, Scottish physician James Lind applied epidemiological observations in identifying the causation of scurvy and suggesting treatment for the disease.
During the period 1848-1854, British anesthetist John Snow conducted a series of classic studies of cholera that led to an understanding of the causation of the disease. Joseph Goldberger of the U.S. Public Health Service worked to identify the causation of pellagra from 1916 to 1918. During the 1920’s, Austin Bradford Hill, an English statistician, studied the difference in mortality between people living in urban settings and those living in rural areas. Later, with his student Richard Doll, he conducted the classic study that established the role of cigarette smoking in causation of lung cancer. In more recent years, widely publicized epidemiological...
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Descriptive Epidemiology (Forensic Science)
Epidemiology can be divided into two categories: descriptive and analytical. Descriptive epidemiology deals with time, place, and person distribution of the health-related state being discussed. One type of study conducted in descriptive epidemiology is the correlational study, in which researchers compare the frequencies of health-related states between different groups during the same time period or in the same population at different time periods. Another type of study in descriptive epidemiology involves case reports and case series, which are descriptive accounts of individual cases. Descriptive epidemiology also produces cross-sectional surveys, which are snapshot accounts that capture both disease exposure and outcome at the same point in time.
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Analytical Epidemiology (Forensic Science)
Analytical epidemiology deals with determinants of health-related states and aims at testing the hypotheses developed from descriptive studies. Its purpose is to find statistical associations to establish cause-effect relationships. Among the types of studies conducted in analytical epidemiology are case-control studies, in which series of persons with a given disease are compared with those without the disease and the exposure of interest is examined. Another type is the cohort study, in which a group of people with the exposure and a group without the exposure are followed over time, and the disease manifestation in the two groups is examined. Intervention trials are also conducted in analytical epidemiology; in such a study, the exposure status of each participant is assigned by the investigator.
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Further Reading (Forensic Science)
Brownson, Ross C., Patrick L. Remington, and James R. Davis. Chronic Disease Epidemiology and Control. 2d ed. Washington, D.C.: American Public Health Association, 1998. Easy-to-read textbook discusses methods in chronic disease epidemiology and addresses the lifestyle risk factors of tobacco use, alcohol use, physical inactivity, diet and nutrition, high blood pressure, and cholesterol.
Goodman, Richard A., et al. “Forensic Epidemiology: Law at the Intersection of Public Health and Criminal Investigations.” Journal of Law, Medicine, and Ethics 31 (2003): 684-700. Seminal article defines the field of forensic epidemiology, summarizes past and current applications of forensic epidemiology, and discusses the joint training for law enforcement and public health officials in forensic epidemiology.
Hennekens, Charles H., and Julie E. Buring. Epidemiology in Medicine. Boston: Little, Brown, 1987. Textbook in epidemiology from a clinical perspective. Basic concepts are presented along with types of epidemiological studies, description and analysis of epidemiological data and epidemiology in disease control.
Koehler, Steven A., Shaun Ladham, Abdulrezzak Shakir, and Cyril H. Wecht. “Simultaneous Sudden Infant Death Syndrome: A Proposed Definition and Worldwide Review of Cases.” American Journal of Forensic Medicine and Pathology 22 (March, 2001): 23-32. Interesting example of...
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Science and Profession (Magill’s Medical Guide, Sixth Edition)
Although epidemiology is closely related to medicine, there are significant differences between the two fields. The main focus of medicine is to diagnose and to treat diseases in individuals, while the core purpose of epidemiology is to identify factors that cause health problems and to control diseases in populations. The health of a population is the responsibility of the field of public health, and epidemiology is a tool for public health. Epidemiology studies disease distribution in populations (for example, how often a disease occurs in different groups of people), examines determinants of diseases or risk factors that increase the risk for disease development, and evaluates strategies to prevent and control diseases in communities.
Diseases have certain patterns in populations. Some groups of people are at a higher risk for a particular disease. For example, smokers are at a higher risk for lung cancer. A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk. The concept of a population at risk can be explained by the traditional epidemiological triangle model. In this model, the three angles are agent, host, and environment. The interrelationship of these three factors is the basis of development of disease in the population.
In the triangle model, the agent is the cause of the disease and includes four main categories: biological, physical, chemical, and...
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Diagnostic and Treatment Techniques (Magill’s Medical Guide, Sixth Edition)
The techniques or methods that epidemiologists use to investigate diseases in populations are epidemiological studies, which mainly consists of cross-sectional studies, case-control studies, and cohort studies.
Cross-sectional studies are also called descriptive epidemiology, because this method describes the distribution of diseases or health-related events and the exposure status of risk factors in terms of person, place, and time. Describing disease distribution by person allows discovery of disease frequency and the populations at greatest risk for the disease. Populations at a high risk can be identified by investigating such characteristics as age, gender, race, education, occupation, income, living arrangement, health status, smoking status, physical activity level, medication use, and access to health care. Disease frequencies can be observed specifically for any of these characteristics by different classifications. For example, hypertension occurrence can be observed by physical activity levels, such as low, medium, and high. Through comparisons of hypertension frequency among the three levels of physical activity, the group with the highest hypertension rate can be identified. Describing disease distribution by place can provide information associated with the geographic extent of the disease. This information includes county, state, country, birthplace, and workplace. Identifying place allows...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
Literally translated from Greek, “epidemiology” means “the study of people”—the population-level study of disease. The origins of epidemiology began with John Snow, a physician of London in the eighteenth century who investigated an epidemic of cholera. By observing and plotting the location of deaths related to the disease, Snow was able to legitimize his finding that cholera was spread through contaminated water and food.
In its early years, epidemiology was mainly used to study epidemics of infectious diseases, because infectious diseases were the major cause of death in populations at that time. Through improvements in nutrition and living standards, as well as advances in medicine, the major cause of death has been shifted from infectious diseases to noninfectious or chronic diseases, accompanied by a longer life expectancy in developed countries such as the United States. Epidemiology has now been applied to chronic diseases and other conditions as well, such as cancer, heart disease, diabetes, and injuries. The Framingham Heart Study is a famous epidemiological study of cardiovascular disease in the U.S. population. Epidemiologic methods have been approved as a powerful tool to study diseases or other conditions in populations and have been also applied to other fields, such as sociology.
In the future, the use of epidemiologic methods will continue to expand and allow an understanding of...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Day, Ian N. M., ed. Molecular Genetic Epidemiology: A Laboratory Perspective. New York: Springer, 2002. This book describes approaches to a series of methodologies in laboratories engaged in molecular and genetic epidemiological studies of population samples. It contains overviews of core topics and techniques that are widely available to researchers.
Fletcher, Robert H., and Suzanne W. Fletcher. Clinical Epidemiology: The Essentials. 4th ed. Baltimore: Lippincott Williams & Wilkins, 2005. Provides students and clinicians with the basic principles and concepts of clinical epidemiology, which helps develop a system observing and assessing outcomes in patients for the improvement of care for future patients.
Last, John M., et al., eds. A Dictionary of Epidemiology. 4th ed. New York: Oxford University Press, 2001. A standard English-language dictionary of epidemiology and many other related fields, including biostatistics, infectious disease control, health promotion, genetics, and medical ethics.
Lilienfeld, David E., and Paul D. Stolley. Foundations of Epidemiology. 3d ed. New York: Oxford University Press, 1994. A textbook commonly used by graduate students of epidemiology, with comprehensive information about concepts and methods of epidemiology. It provides numerous classical epidemiological study examples.
Newman, Stephen C. Biostatistical Methods in...
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Epidemiology (Encyclopedia of Public Health)
Epidemiology is the indispensable basic science of public health. It provides the logical framework for the facts that enable public health officials to identify important public health problems and to delineate their dimensions. Epidemiologic methods are used to define these health problems; to classify, identify, and elucidate their causes; and to plan and evaluate rational control measures.
HISTORICAL DEVELOPMENT OF EPIDEMIOLOGY
In ancient times, epidemics and plagues were terrifying natural phenomena that cried out for a more rational explanation than that they were due to the wrath of god or the machinations of evil spirits. Hippocrates (c. 46077 B.C.E.) described many kinds of epidemics and in On Airs, Waters, Places and other writings. He offered empirical insights into environmental and behavioral factors that might be associated with certain kinds of disease. Although doctors and others engaged in the healing arts did not clearly understand the concept of contagion until several hundred years later, Fracastorius (c. 1478553) identified several ways that infections can be transmittedy direct contact, by what we now call droplet spread, and by contaminated clothing.
The science of epidemiology took root with empirical observations of epidemics and other causes of death. John Graunt (1620674), in London, complied the first mortality tables on England's bills of mortality. Statistical analyses of deaths due to childbed fever by Ignaz Semmelweiss (1818865) in Vienna in the early nineteenth century and of tuberculosis by Pierre Charles Alexandre Louis (1787872) in Paris demonstrated the power of numbers. In London, in 1848 and 1854, meticulous, logical examination of the facts and figures about cholera epidemics by John Snow (1813858) revealed the mode of communication of this deadly epidemic disease. Snow is regarded as the founder of modern epidemiology because of his use of such careful methods.
Until early in the twentieth century almost all epidemiology focused on communicable diseases, although Percivall Pott's (1714788) observations on cancer of the scrotum in chimney sweeps and James Lind's dietary experiment with fresh fruit to prevent scurvy (1975) were precursors of modern noncommunicable disease epidemiology and clinical trials, respectively. The use of epidemiology in studies of coronary heart disease and cancer in large-scale trials of many new preventive and therapeutic regimens, in nationwide surveys of health status, and in evaluation of health services came to the fore in the second half of the twentieth century. In the final quarter of the twentieth century, powerful computers, information technology, and more rigorous methodological approaches transformed epidemiology and made it a mandatory feature of clinical science as well as the most fundamental basic science of public health.
DEFINITION AND SCOPE
The word "epidemiology" was coined in the midnineteenth century to describe the scientific study of epidemics. Its meaning has expanded over the years, and present-day epidemiology encompasses the study of all varieties of illness and injury as they affect defined groups of people. In 1983 a committee representing the International Epidemiological Association defined epidemiology as "the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems." Study includes observation, surveillance, hypothesis-testing research projects, analysis of epidemiologic and other kinds of data, and certain other kinds of experiments. Distribution includes analysis of data according to the time scale over which events occur, the places where the events occur, and the categories of persons to whom they occur. Determinants are all the physical, biological, behavioral, social, and cultural factors that influence health. Health-related states or events include diseases, causes of death, behaviors such as the use of tobacco, reactions to preventive regimens, and provision and use of health services. Specified populations are those with identifiable characteristics such as known numbers and age groups. The ultimate aim and purpose of epidemiologyo promote, protect, and restore good healths manifested in the "application of this study to control health problems."
Epidemiologists attempt to identify, measure, count, and control diseases, injuries, and causes of untimely death; and to relate these events to the associated inherited, environmental, and behavioral factors that cause or contribute to them. One of the great intellectual challenges of epidemiology is to dissect these factors and unravel their connections in order to identify exactly what is ultimately responsible for a particular disease or health problem.
RELATIONSHIP TO OTHER SCIENCES AND TECHNOLOGIES
The information used by epidemiologists comes from a diverse array of sources; draws on a wide range of sciences and technologies; and calls on the expertise of technologists and other people engaged in many kinds of crafts. Some connections are obvioushose with vital statistics, biostatistics, microbiology, immunology, and chemistry; with every clinical specialty from pediatrics to geriatrics and palliative care, and from family practice to hematology and neurosurgery. Other obvious connections are to the social and behavioral sciences, and, less obviously, to animal husbandry, wildlife biology, agricultural science, physics, atmospheric sciences, oceanography, engineering, town planning, education, law enforcement, communications technology, and the media. Epidemiology may be the most ecumenical of all the sciences. Probably no other branch of biomedical science has so many connections to such a wide range of other human activities.
The basis of all epidemiology is the comparison of groups of people. For these comparisons to be valid, it is necessary to convert raw numbers into rates. A rate is a fractionhe upper part (the numerator) is the number of people affected by the problem, event, or condition of interest; the lower part (the denominator) is the number of persons in the population who are at risk of experiencing the problem, event, or condition. Because the events normally continue over a long period, often indefinitely, rates are expressed in relation to a specified time. Since fractions are awkward to deal with, there is commonly a multiplier, and the rate, as shown in the following formula, is expressed in terms of so many per thousand, per hundred thousand, etc., in a specified time, usually a year, though shorter periods are used when circumstances warrant it:
In practice there are many variations in the ways rates are expressed, but the basic elements of events, population at risk, and time are common to all.
Rates have many uses. By comparing rates, epidemiologists can examine the experience of particular groups of people at specified times, in different cities, countries, or occupational groups. The observed differences are the basis for inferences about the reasons for these differences, and are used to test hypotheses about these reasons, possibly about the putative cause of a particular kind of cancer, for instance. In addition to the absolute requirement, for validity, of basing all comparisons on rates, another important use is in calculating the risks to individuals and groups of experiencing an event such as a heart attack, the occurrence of cancer, or traffic injury. Comparisons are often rendered invalid, or relatively unreliable, by differences among the populations being comparedften because of failure to allow for various kinds of biases and confounding factors. A common problem stems from differences in the age composition of populations that are being compared. This problem is overcome by the procedure of age-adjustment. Another problem is that there may be important qualitative differences, such as health or employment status, between groups that are being compared.
The terms "incidence" and "prevalence" are often confused. Incidence refers to the number of new cases, events, or deaths, that occur in a specified time, usually one year. Prevalence refers to the total number of events or cases, both new and long-term, that are present at a particular point in time. Prevalence is therefore expressed as a number, not a rate, as there is no time dimension involved.
An epidemic is the occurrence of a number of cases of a disease clearly in excess of normal expectation. This is usually a large number when the disease is one of the common infectious fevers, but even a single case of a dangerous contagious disease, such as typhoid, that has long been absent from a community should suffice to activate the highest level of epidemic surveillance and control measures. The occurrence of a small number of cases of a rare variety of cancer, closely clustered in time and space, may also signal an epidemic. Observational and analytic epidemiology blend in the investigation of epidemics. The investigation demands meticulous attention to detail in collecting information about all the cases of the condition, including mild and inconspicuous cases as well as those with florid manifestations, and must include details about all possible associated factors, such as dietary intake (this is especially important in outbreaks of food poisoning), occupation, living conditions, and unusual recent experiences. Particular attention is paid to the index casehe first identified case of a condition. In most infectious disease epidemics, this could be the case that introduced the infection into the affected community. Information is also gathered about healthy people in the same community, aimed at discovering why they have not been affected. Laboratory tests are used to confirm the diagnosis, identify the pathogenic organism, toxic chemical, or other agent that caused the disease; and to measure immunological responses among both sick and healthy people. Analyzing all this information often clarifies the nature and cause of an epidemic and points the way to appropriate control measures.
Investigating epidemics can be tedious because it needs to be so painstaking, even, seemingly, a boring routine task. But often it is as exciting as detective fiction. For example, an epidemic of typhoid in Aberdeen, Scotland, was traced eventually to a contaminated can of processed beef from Argentina. The can had been cooled in a river adjacent to the canning works. As the pressure inside the can fell when it cooled, a partial vacuum was created and typhoid bacilli in raw sewage in the water were sucked into the can through a minute hole.
Identifying the existence of an epidemic sometimes requires unusual vigilance and an ability to make connections among seemingly isolated events. An epidemic of lethal pneumonia among members of the American Legion who attended a convention in Philadelphia in 1976 and then returned to their hometowns before becoming ill, would not have come to light without rigorous scrutiny on the part of epidemic intelligence service officers of the Centers for Disease Control. Subsequent investigations led to the identification of Legionnaire's disease.
Techniques of molecular biology, notably DNA typing and the identification of biomarkers, have immensely enhanced the precision of epidemic investigation. It is now possible to trace the exact passage of an infectious agent such as the gonococcus or HIV (human immunodeficiency virus) as it is transmitted by direct contact from one individual to another among a group of people; or to show that coughing by a passenger with open pulmonary tuberculosis on a crowded airline flight can cause primary tuberculous infection of other passengers in the same compartment of that flight; or to determine how certain cancer-causing agents actually induce cancer. Books and articles in the popular press, notably the accounts by the journalist Berton Roueché in the New Yorker, and on some TV programs have communicated the excitement and challenge of epidemic investigations.
The application of several analytic methods of epidemiologic study has contributed substantially to scientists' understanding of disease causation, and therefore to control and prevention of many conditions of great public health importance. The available methods are observational epidemiology (the empirical study of naturally occurring events), analytic study, and, under carefully defined conditions and with all due ethical safeguards, human experimentation.
Observational Epidemiology. This method begins with surveillance of populations, using vital and health statisticsncluding analysis of death rates arranged by age, sex, locality, and cause of death. Other information is derived from notified cases of infectious diseases of public health importance, from registries of cancer or other diseases, and from hospital discharge statistics. Since 1957, the National Center for Health Statistics has conducted continuously a National Health Survey that has carried observational epidemiology to new levels of comprehensiveness.
It is often possible to make imaginative use of many other kinds of available information about defined population groups. Schools and many employers keep records of absences due to sickness, sometimes with reasons for these absences. Police and other law enforcement agencies keep records of calls to settle domestic disputes and of damage due to vandalism, which are useful indicators of social pathologies associated with local variations in the frequency of domestic violence, alcohol abuse, and broken families. All such sources of information combine to make it possible for epidemiologists and public health specialists to produce a multidimensional "community diagnosis." Serial measurements can indicate whether things are improving or getting worse, and in which ways these trends are moving for each of different indicators ranging from adolescent smoking behavior to reasons for long-term disability among the elderly.
Analytic Observational Studies. The possibilities of observational epidemiology are considerable, but not limitless. They are powerfully reinforced by analytic studies. The two main analytic methods are the case-control study and the cohort study.
Careful questioning of patients has enabled many doctors to make inferences about the influence of past experience on present disease. Percivall Pott, an eighteenth-century British physician, observed that cancer of the scrotum occurred among former chimney sweeps, and correctly inferred that it was associated with the accumulation of tar in the skin creases. Two hundred years later, in 1940, Norman Gregg, an ophthalmologist in Sydney, Australia, similarly inferred correctly that the cases he was seeing of congenital cataract must be associated with rubella (German measles), which their mothers had had during early pregnancy.
The case-control study is a systematic extension of routine medical history taking, in which the past histories of patients (the cases) suffering from the condition of interest are compared to the past histories of persons (the controls) who do not have the condition of interest, but who otherwise resemble the cases in such particulars as age and sex. Analysis of data about a series of cases and controls may show differences that are statistically significant. Sometimes only small numbers of cases are required to demonstrate significant differences between cases and controls. This makes the case-control study a suitable way to search for causes of rare conditions. For example, the discovery that a very rare form of liver cancer was strongly associated with occupational exposure to vinyl chloride required only four cases, and the fact that expectant mothers' use of artificial estrogens during early pregnancy can cause cancer of the vagina many years later in their daughters was based on a case-control study of eight cases. Although case-control studies can be flawed by the presence of biases that are often difficult or even impossible to eliminate, they are a valuable method of investigation because they can be done rapidly and at relatively little expense. The findings can be confirmed or refuted by more rigorous research methods such as cohort studies.
A cohort study is conducted by identifying individuals in a defined population who are exposed to varying levels of known or suspected risk for the condition of interest, such as cancer of the lung or coronary heart disease. The population is observed over a certain period, and the death and disease incidence rates among those exposed to varying and known levels of risk are compared. Cohort studies require large numbers, commonly many thousands, and prolonged observation, commonly years or even decades. They are therefore expensive, requiring a large and dedicated staff and maintenance of detailed records of very large numbers of people, only a small proportion of whom will ultimately fall ill and die of the condition of interest. Some cohort studies have become famous. The people of Framingham, Massachusetts, have been the subjects of cohort studies of coronary heart disease since 1948. In 1951, Richard Doll and Austin Bradford Hill began a cohort study of lung cancer in relation to tobacco smoking in a cohort of about 40,000 male British doctors. Later phases of this study have expanded to include risk factors for coronary heart disease and other chronic conditions; and by the late 1990s this study had yielded dramatic evidence of the relationship of tobacco smoking to cancers of many kindsnd to coronary heart disease, chronic obstructive lung disease, and various other life-shortening chronic diseases.
It is possible to get results from a cohort study without waiting many years, if detailed information about exposure to risk factors at some time in the past is available in sufficient detail for a population of sufficient size. A method that permits reliable linking of past and present medical and other relevant records, such as a record linkage system, facilitates this approach. Record linkage is the process of relating information from two or more sets of recordsompiled years apart and sometimes by different agenciesbout the same individuals. A prerequisite is a way to identify individuals with a high degree of precision, such as a unique numbering system, or a system combining a sequence of digits for birthdate, birthplace, and sex; with alphabet letters or a phonetic code used for other details, such as the individual's mother's maiden name. Obviously the logistics of all this make it a costly method, but the yield can justify the expense. This method, known as an historical cohort study, has demonstrated the relationship of childhood cancer and developmental anomalies to prenatal maternal exposure to small diagnostic doses of X-rays. Record linkage and historical cohort studies have also demonstrated a relationship between birthweight and the occurrence of cardiovascular disease in middle age.
Experimental Epidemiology. In the 1920s, experimental epidemiology meant observing the passage of infectious pathogens in colonies of rodents, but such experiments are rarely necessary, and the meaning of the term has changed. Experiments in which the investigator studies the effects of intentional alteration or intervention in the course of a disease are now done on humans rather than experimental animals, usually using a randomized controlled-trial design.
The randomized controlled trial (RCT) is a form of human experimentation in which the subjects, usually patients, are randomly allocated to receive either a standard accepted therapeutic or preventive regimen, or an experimental regimen. The purpose of random allocation is to eliminate or minimize bias in the selection of subjects. This greatly enhances the validity of the results. Preferably, the subjects and those observing the trial's results should be unaware of which subjects are receiving the experimental and control regimens, thus eliminating the power of suggestion as a factor influencing the response of individuals to the regimen. There are very important ethical constraints on the conduct of randomized controlled trials. The only ethically acceptable justification for conducting a randomized controlled trial is uncertainty about which of the available regimens is the best, a state of affairs known as "equipoise." It is absolutely essential to obtain the genuinely informed consent of all human subjects on whom a trial is conducted.
CLINICAL EPIDEMIOLOGY AND EVIDENCE-BASED MEDICINE
In the final quarter of the twentieth century, physicians in clinical practice discovered the value of epidemiologic methods in enhancing the efficacy of treatment regimens, mainly through rigorous attention to the nature and quality of the evidence on which clinical decisions are based. Evidencebased medicine then moved into public health practice, where it is illuminating decisions about many aspects of public health practice, such as the most effective way to deploy public health nurses in a local health department.
OTHER RECENT ADVANCES
Epidemiology made spectacular progress in several other directions in the 1990s. One was in the application of molecular biology, resulting in what is sometimes called molecular epidemiology. Other advances have been made in genetic epidemiology, where the meeting of molecular genetics with public health, occupational and environmental health, and infant and child health has produced both exciting stories of great progress and difficult ethical and moral problems. What are scientists and physicians to do, for instance, with the newfound knowledge and technical capability to identify defective genes, especially genes that, in interaction with some environmental circumstances, can disqualify certain individuals from particular occupations and can render others ineligible for life insurance? Such dilemmas presage a testing time for society's values.
Another set of new challenges face epidemiologists who specialize in studies of risk management. The global environment is changing as the burden of greenhouse gases increases and leads to a rise in average global ambient temperatures, and remote sensing and climate models enable us to predict the likely future distribution of vector-borne diseases such as malaria, dengue, and schistosomiasis. A new realm of risk factor analysis is thus emerging, based on future health scenarios that incorporate climate models andin the most sophisticated applicationsnclude sets of models for future patterns of biodiversity, human settlements, and economic and industrial dynamics. In these ways epidemiologists are helping to plan the public health services that will be needed in the future.
JOHN M. LAST
(SEE ALSO: Case-Control Study, Cohort Study, Cross-Sectional Study; Epidemiologic Transition; Graunt, John; Hippocrates of Cos; Mortality Rates; Notifiable Diseases; Pott, Percivall; Rates; Rates: Age-Adjusted; Record Linkage; Semmelweiss, Ignaz; Snow, John; Vital Statistics; and other articles on specific diseases mentioned herein)
Ashton, J., ed. (1994). The Epidemiological Imagination. Buckingham, UK: Open University Press.
Beaglehole, R.; Bonita, R.; and Kjellström, T. (1993). Basic Epidemiology. Geneva: World Health Organization.
Buck, C.; Llopis, A.; Nájera, E.; and Terris, M., eds. (1988). The Challenge of Epidemiology. Washington, DC: Pan American Health Organization.
Last, J. M., ed. (2000). A Dictionary of Epidemiology, 4th edition. New York: Oxford University Press.
Rothman, K. J., and Greenland, S., eds. (1998). Modern Epidemiology, 2nd edition. Philadelphia: Lippincott-Raven.
Roueché, B. (1954). Eleven Blue Men, and Other Narratives of Medical Detection. Boston: Little, Brown & Co.
Epidemiology (World of Microbiology and Immunology)
Epidemiology is the study of the various factors that influence the occurrence, distribution, prevention, and control of disease, injury, and other health-related events in a defined human population. By the application of various analytical techniques including mathematical analysis of the data, the probable cause of an infectious outbreak can be pinpointed. This connection between epidemiology and infection makes microorganisms an important facet of epidemiology.
Epidemiology and genetics are two distinct disciplines that converge into a new field of human science. Genetic epidemiology, a broad term used for the study of genetics and inheritance of disease, is a science that deals with origin, distribution, and control of disease in groups of related individuals, as well as inherited causes of diseases in populations. In particular, genetic epidemiology focuses on the role of genetic factors and their interaction with environmental factors in the occurrence of disease. This area of epidemiology is also known as molecular epidemiology.
Much information can come from molecular epidemiology even in the exact genetic cause of the malady is not known. For example, the identification of a malady in generations of related people can trace the genetic characteristic, and even help identify the original source of the trait. This approach is commonly referred to as genetic screening. The knowledge of why a particular malady appears in certain people, or why such people are more prone to a microbial infection than other members of the population, can reveal much about the nature of the disease in the absence of the actual gene whose defect causes the disease.
Molecular epidemiology has been used to trace the cause of bacterial, viral, and parasitic diseases. This knowledge is valuable in developing a strategy to prevent further outbreaks of the microbial illness, since the probable source of a disease can be identified.
Furthermore, in the era of the use of biological weapons by individuals, organizations, and governments, epidemiological studies of the effect of exposure to infectious microbes has become more urgently important. Knowledge of the effect of a bioweapon on the battlefield may not extend to the civilian population that might also be secondarily affected by the weapons. Thus, epidemiology is an important tool in identifying and tracing the course of an infection.
The origin of a genetic disease, or the genetic defect that renders someone more susceptible to an infection (e.g., cystic fibrosis), can involve a single gene or can be more complex, involving more than one gene. The ability to sort through the information and the interplay of various environmental and genetic factors to approach an answer to the source of a disease outbreak, for example, requires sophisticated analytical tools and personnel.
Aided by advances in computer technology, scientists develop complex mathematical formulas for the analysis of genetic models, the description of the transmission of the disease, and genetic-environmental interactions. Sophisticated mathematical techniques are now used for assessing classification, diagnosis, prognosis and treatment of many genetic disorders. Strategies of analysis include population study and family study. Population study must be considered as a broad and reliable study with an impact on public health programs. They evaluate the distribution and the determinants of genetic traits. Family study approaches are more specific, and are usually confirmed by other independent observations. By means of several statistical tools, genetic epidemiologic studies evaluate risk factors, inheritance and possible models of inheritance. Different kinds of studies are based upon the number of people who participate and the method of sample collection (i.e., at the time of an outbreak or after an outbreak has occurred). A challenge for the investigator is to achieve a result able to be applied with as low a bias as possible to the general population. In other words, the goal of an epidemiological study of an infectious outbreak is to make the results from a few individuals applicable to the whole population.
Such analytical tools and trained personnel are associated more with the developed world, in the sense that expensive analytical equipment and chemicals, and highly trained personnel are required. However, efforts from the developed world have made such resources available to under-developed regions. For example, the response of agencies such as the World Health Organization to outbreaks of hemorrhagic fevers that occur in underdeveloped regions of Africa can include molecular epidemiologists.
A fundamental underpinning of infectious epidemiology is the confirmation that a disease outbreak has occurred. Once this is done, the disease is followed with time. The pattern of appearance of cases of the disease can be tracked by developing what is known as an epidemic curve. This information is vital in distinguishing a natural outbreak from a deliberate and hostile act, for example. In a natural outbreak the number of cases increases over time to a peak, after which the cases subside as immunity develops in the population. A deliberate release of organisms will be evident as a sudden appearance of a large number of cases at the same time.
Analysis of a proper sample size, as well as study type are techniques belonging to epidemiology and statistics. They were developed in order to produce reliable information from a study regarding the association of genetic and environmental factors. Studies that are more descriptive consider genetic trait frequency, geographic distribution differences, and prevalence of certain conditions in different populations. On the other hand, studies that analyze numerical data consider factors like association, probability of occurrence, inheritance, and identification of specific groups of individuals.
Thus, molecular epidemiology arises from varied scientific disciplines, including genetics, epidemiology, and statistics. The strategies involved in genetic epidemiology encompass population studies and family studies. Sophisticated mathematical tools are now involved, and computer technology is playing a predominant role in the development of the discipline. Multidisciplinary collaboration is crucial to understanding the role of genetic and environmental factors in disease processes.
See also Bacteria and bacterial infection; Genetic identification of microorganisms; History of microbiology; History of public health; Infection control; Public health, current issues; Transmission of pathogens
Epidemiology (World of Forensic Science)
Epidemiology is the study of the occurrence, frequency, and distribution of diseases in a given population. As part of this study, epidemiologistscientists who investigate epidemics (widespread occurrence of a disease that occurs during a certain time)ttempt to determine how the disease is transmitted, and what are the host(s) and environmental factor(s) that start, maintain, and/or spread the epidemic.
Epidemiology can be an important facet of a forensic investigation. A recent infamous example occurred in the fall of 2001, when a number of letters containing spores of Bacillus anthracis, the agent that causes anthrax, were sent through the United States postal system. The illnesses and deaths that resulted prompted the near shut-down of the postal delivery system, and an investigation to find the sender(s) of the letters and the source of the bacterial spores. These investigations were rooted in epidemiology.
The primary focus of epidemiology is on groups of persons, rather than individuals. The primary effort of epidemiologists is in determining the etiology (cause) of the disease and identifying measures to stop or slow its spread. This information, in turn, can be used to create strategies by which the efforts of health care workers and facilities in communities can be most efficiently allocated for this purpose.
In tracking a disease outbreak, epidemiologists may use any or all of three types of investigation: descriptive epidemiology, analytical epidemiology, and experimental epidemiology.
Descriptive epidemiology is the collection of all data describing the occurrence of the disease, and usually includes information about individuals infected, and the place and period during which it occurred. Such a study is usually retrospective, i.e., it is a study of an outbreak after it has occurred. The 2001 anthrax investigation is one example.
Analytical epidemiology attempts to determine the cause of an outbreak. Using the case control method, the epidemiologist can look for factors that might have preceded the disease. Often, this entails comparing a group of people who have the disease with a group that is similar in age, sex, socioeconomic status, and other variables, but does not have the disease. In this way, other possible factors, e.g., genetic or environmental, might be identified as factors related to the outbreak.
Using the cohort method of analytical epidemiology, the investigator studies two populations, one who has had contact with the disease-causing agent and another that has not. For example, the comparison of a group that received blood transfusions with a group that has not might disclose an association between blood transfusions and the incidence of a blood borne disease, such as hepatitis B.
Experimental epidemiology tests a hypothesis about a disease or disease treatment in a group of people. This strategy might be used to test whether or not a particular antibiotic is effective against a particular disease-causing organism. One group of infected individuals is divided randomly so that some receive the antibiotic and others receive a placebo "false" drug that is not known to have any medical effect. In this case, the antibiotic is the variable, i.e., the experimental factor being tested to see if it makes a difference between the two otherwise similar groups. If people in the group receiving the antibiotic recover more rapidly than those in the other group, it may logically be concluded that the variablentibiotic treatmentade the difference. Thus, the antibiotic is effective.
In the process of studying the cause of an infectious disease, epidemiologists often view it in terms of the agent of infection (e.g., particular bacterium or virus), the environment in which the disease occurs (e.g., crowded slums), and the host (e.g., hospital patient). Another way epidemiologists may view etiology of disease is as a "web of causation." This web represents all known predisposing factors and their relations with each other and with the disease. For example, a web of causation for myocardial infarction (heart attack) can include diet, hereditary factors, cigarette smoking, lack of exercise, susceptibility to myocardial infarction, and hypertension. Each factor influences and is influenced by a variety of other factors.
Epidemiologic investigations are largely mathematical descriptions of persons in groups, rather than individuals. The basic quantitative measurement in epidemiology is a count of the number of persons in the group being studied who have a particular disease; for example, epidemiologists may find 10 members of a village in the African village of Zaire suffer from infection with Ebola virus infection; or that 80 unrelated people living in an inner city area have tuberculosis.
A fundamental underpinning of infectious epidemiology is the confirmation that a disease outbreak has occurred. Once this is done, the disease is followed with time. The pattern of appearance of cases of the disease can be tracked by developing what is known as an epidemic curve. This information is vital in distinguishing a natural outbreak from a deliberate and hostile act, for example. The appearance of a few cases at first with the number of cases increasing over time to a peak is indicative of a natural outbreak. The number of cases usually begins to subside as the population develops immunity to the infection (e.g., influenza). However, if a large number of cases occur in the same area at the same time, the source of the infection might not be natural. Examples include a food poisoning or a bioterrorist action where the accidental or deliberate release of organisms will be evident as a sudden appearance of a large number of cases at the same time.
Any description of a group suffering from a particular disease must be put into the context of the larger population. This shows what proportion of the population has the disease. The significance of ten people out of a population of 1,000 suffering tuberculosis is vastly different, for example, than if those ten people were part of a population of one million.
Thus one of the most important tasks of the epidemiologist is to determine the prevalence ratehe number of persons out of a particular population who have the disease (prevalence rate). A prevalence rate can represent any time period, e.g., day or hour; and it can refer to an event that happens to different persons at different times, such as complications that occur after drug treatment (on day five for some people or on day two for others).
The incidence rate is the rate at which a disease develops in a group over a period of time. Rather than being a snapshot, the incidence rate describes a continuing process that occurs over a particular period of time.
Period prevalence measures the extent to which one or all diseases affects a group during the course of time, such as a year.
Epidemiologists also measure attributable risk, which is the difference between two incidence rates of groups being compared, when those groups differ in some attribute that appears to cause that difference. For example, the lung cancer mortality rate among a particular population of non-smoking women 50 to 70 years old might be 20/100,000, while the mortality rate among woman in that age range who smoke might be 150/100,000. The difference between the two rates (150 20 = 130) is the risk that is attributable to smoking, if smoking is the only important difference between the groups regarding the development of lung cancer.
Epidemiologists arrange their data in various ways, depending on what aspect of the information they want to emphasize. For example, a simple graph of the annual occurrence of viral meningitis might show by the "hills" and "valleys" of the line in which years the number of cases increased or decreased. This might provide evidence of the cause and offer ways to predict when the incidence might rise again.
Bar graphs showing differences in rates among months of the year for viral meningitis might pinpoint a specific time of the year when the rate goes up, for example, in summertime. That, in turn, might suggest that specific summertime activities, such as swimming, might be involved in the spread of the disease.
One of the most powerful tools an epidemiologist can use is case reporting: reporting specific diseases to local, state, and national health authorities who accumulate the data. Such information can provide valuable leads as to where, when, and how a disease outbreak is spread, and help health authorities to determine how to halt the progression of an epidemicne of the most important goals of epidemiology.
Molecular epidemiology has been used to trace the cause of bacterial, viral, and parasitic diseases. This knowledge is valuable in developing a strategy to prevent further outbreaks of the microbial illness, since the probable source of a disease can be identified.
Molecular epidemiology arises from varied scientific disciplines, including genetics, epidemiology, and statistics. The strategies involved in genetic epidemiology encompass population studies and family studies. Sophisticated mathematical tools are now involved, and computer technology is playing a predominant role in the development of the discipline. Multidisciplinary collaboration is crucial to understanding the role of genetic and environmental factors in disease processes.
Much information can come from molecular epidemiology, even in the exact genetic cause of the malady is not known. For example, the identification of a malady in generations of related people can trace the genetic characteristic, and even help identify the original source of the trait. This approach is commonly referred to as genetic screening. The knowledge of why a particular malady appears in certain people, or why such people are more prone to a microbial infection than other members of the population, can reveal much about the nature of the disease in the absence of the actual gene whose defect causes the disease.
Various routes can spread infections (i.e., contact, air borne, insect borne, food and water intake, etc.). Likewise, the route of entry of an infectious microbe can also vary from microbe to microbe.
Laboratory analysis techniques can be combined with other techniques to provide information related to the spread of an outbreak. For example, microbiological data can be combined with geographic
Reconstructing the movements of people is especially important when the outbreak is of an infectious disease. The occurrence of the disease over time can yield information as to the source of an outbreak.
Epidemiologists were among the first scientists to effectively utilize the Internet and email capabilities to effectively communicate regarding disease outbreaks. The International Society for Infectious Diseases sponsors PROMED, a global e-mail based electronic reporting system for outbreaks of emerging infectious diseases and toxins, which is open to all sources.
SEE ALSO Anthrax, investigation of 2001 murders; Ebola virus; Pathogens; September 11, 2001, terrorist attacks (forensic investigations of).