Encyclopedia of Public Health | Epidemics

An epidemic is an occurrence of cases of a disease in excess of usual expectations for a particular population. An outbreak of influenza that affects thousands of people in a month in a nation and a half dozen cases of a rare form of liver cancer affecting industrial workers in a chemical plant over a period of several years are both examples of epidemics. Another kind of epidemic can be seen in the sharp rise in the prevalence of cigarette smoking throughout the twentieth century—first among males and then females—and of smoking-related respiratory system cancers. The surging death rate from coronary heart disease among men in many industrial nations in the middle third of the twentieth century may also be described as an epidemic.

A pandemic is a worldwide epidemic that kills or incapacitates huge numbers in many countries. Outbreaks of influenza in 1919 and HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome) since the 1980s are both examples of pandemics. Conditions that are constantly present in a community are called "endemic;" examples include malaria in some tropical regions, and goiter due to deficiency of iodine in the soil of certain areas.

A single case of a rare and dangerous contagious disease that has never occurred before or has long been absent from a community represents a potential epidemic, as does a small cluster of cases of a disease such as typhoid in an urban community with good sanitation. Infectious pathogens (bacteria and viruses) cause most epidemics, while some are caused by a toxic industrial process or a toxic substance in food or water. A toxin in cooking oil in Spain in 1981 poisoned several thousand people, damaging their kidneys, liver, lungs, and nervous system and causing many deaths and widespread chronic disability. The precise nature of this contaminant was never established. In 1976, members of the American Legion who had attended a convention in Philadelphia began to fall ill and die of an unusual form of pneumonia, mostly after they returned to their homes elsewhere in the United States. Investigations by the Centers for Disease Control revealed this to be an epidemic of what is now called Legionnaire's disease, which is caused by a previously unknown microorganism that can be disseminated via the moist air in poorly maintained air-conditioning systems.

Charles Mackay, in his classic work Extraordinary Popular Delusions and the Madness of Crowds, described what is known as a behavioral epidemic. This phenomenon can be seen in the reactions of impressionable teenagers at a rock concert—and in a more sinister form in movements such as Nazism, when an entire nation is gripped by destructive fanaticism. The huge increase in traffic-related death and injury rates during the twentieth century, which has continued into the twenty-first century, is a behavioral epidemic associated with addiction to high-speed automobiles (the phenomenon called "road rage" is a psychopathic variation of this epidemic).

Human history has been punctuated frequently by epidemics, and occasionally by pandemics, that have shaped the rise and fall of civilizations and the victories and defeats of warring armies. The outcome of the Peloponnesian War (431–404 B.C.E.) between Athens and Sparta—and the future course of Western civilization—might have been very different had it not been for the epidemic that decimated the Athenians at the beginning of the war. Although the historian Thucydides, who had the disease himself, described its symptoms and signs in detail, modern epidemiologists cannot identify it.

Epidemic sweating sickness recurred several times in medieval Europe, but it has vanished since. The Black Death, or plague, that struck Europe in 1347 killed between one-third and one-half of the people in many cities and towns, arresting the advance of civilization for several generations. Some epidemic diseases, such as the plague, smallpox, typhus, and influenza, have persisted throughout recorded history. Smallpox was eradicated worldwide by 1980. Cholera appeared along the world's major trade routes in several devastating epidemics beginning in the eighteenth century, and it still causes massive epidemics, most recently in South America in early 1990s.

In the final quarter of the twentieth century over thirty new infectious pathogens were identified. Many of these have caused deadly localized epidemics (e.g. Ebola virus, hantavirus, and other viral hemorrhagic fevers), and some have spread worldwide—HIV/AIDS being the foremost among these. Since its first recognition in 1981, HIV has affected almost 40 million people and killed over 10 million, making it the most lethal and dangerous pandemic since the Black Death. Other new and emerging infections that have caused epidemics include Legionnaire's disease, Lyme disease, newly identified hepatitis viruses spread in epidemic form through contaminated blood and blood products used in transfusion services, and several bacterial and viral diseases affecting the gastrointestinal tract.

An epidemic is a public health emergency requiring immediate investigation. The steps in investigating an epidemic are as follows:

1. Confirm the diagnosis.
2. Verify that the number of cases is outside normal expectations.
3. Define features in common among the cases (including inapparent cases).
4. Distinguish cases from members of the community who are not affected.
5. Compare the exposure history of the cases with a sample of noncases.
6. Conduct appropriate laboratory tests for pathogenic organisms.
7. Review environment and social conditions.
8. Arrange, classify, and analyze the data.
9. Plot graphs of time trends and the number of cases; create maps of the distribution of cases.
10. Report findings to the public health authorities for action to control the epidemic.

In the investigation it is important to consider the host (the affected individuals), the agent (the cause of the condition), and the environment. Physical, biological, social, behavioral, and cultural factors must also be considered. Investigating an epidemic can be as exciting as detective fiction, and such investigations (both real and fictional) have yielded many best-selling books and movies. The Epidemic Intelligence Service (EIS) of the U.S. Centers for Disease Control and Prevention has an illustrious record of successfully investigating and controlling epidemics, including some great public health importance. The first investigations of HIV/AIDS were done mainly by EIS staff and close collaborators in New York and Los Angeles.

Several kinds of epidemics can be distinguished. A point-source epidemic is one in which a group of people all fall ill as a result of a single exposure, typically to an agent in food they have all consumed. An example would be an outbreak of acute food poisoning due to staphylococcal enterotoxin. A common-vehicle epidemic is due to an agent that is spread on an ongoing basis in a "vehicle" such as food, water, or air. Food-borne common-vehicle epidemics usually cause gastrointestinal disease, and are sometimes perpetuated by a carrier who is a foodhandler. Waterborne epidemics include typhoid, giardia, viral hepatitis A, and many others. The best known airborne common vehicle epidemic is Legionnaire's disease. Notorious blood-borne common-vehicle epidemics have occurred since the 1980s in many countries after the blood supply became infected with HIV or Hepatitis C virus. Vector-borne epidemics are spread by insect vectors and include viruses such as dengue and viral encephalitis, which are transmitted by mosquitoes.

Control and prevention of an epidemic requires elimination of the source, or, if this is not feasible, precautions to prevent transmissions from the source to susceptible human hosts. The same approach applies when the agent causing the epidemic is not an infectious pathogen but a chemical poison or an allergen; and it can even be applied, with suitable adjustments, to control of behavioral epidemics like mass hysteria and schoolyard vandalism.

Viewed from the perspective of evolutionary biology, epidemics will forever be a part of human-kind's experience. The interaction of human hosts with infectious pathogenic organisms is everchanging, in complex ecosystems that are also ever-changing, often as a result of human activity.

JOHN M. LAST

(SEE ALSO: Adherence or Compliance Behavior; Black Death; Blood-Borne Diseases; Centers for Disease Control and Prevention; Classification of Disease; Common Vehicle Spread; Communicable Disease Control; Contagion; Emerging Infectious Diseases; Epidemic Intelligence Service; Epidemiologic Surveillance; Epidemic Theory: Herd Immunity; Epidemiology; Food-Borne Diseases; HIV/AIDS; Notifiable Diseases; Vector-Borne Diseases; Waterborne Diseases; and articles on specified diseases mentioned herein)