What is virulence?

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The term “virulence” means the disease-producing (pathogenic) capabilities and mechanisms of a microorganism and also the inherent potential of an infection to cause harm.
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The term “virulence” means the disease-producing (pathogenic) capabilities and mechanisms of a microorganism and also the inherent potential of an infection to cause harm.

Disease-Causing Agents

The human body is populated by a multitude of bacteria, viruses, fungi, and parasites. Most pathogens, except viruses, are harmless, and some are even beneficial. These microorganisms live, feed, and grow in or on the body, the host. Microorganisms that take from the host or change it in some way, that contribute nothing to the host’s survival, and that harm the host by causing infection, are known collectively as pathogens or parasites. These abilities, mechanisms, and potentials define a pathogen’s virulence.

Infection virulence is characterized by its effect on the host and is measured by degrees. For example, an infection that causes death is considered more virulent than an infection that causes disability. Mortality rate is another way to measure the virulence of an infection.

The properties and mechanisms of virulence in many pathogens are well known to medical biologists. By understanding the capacity of virulence in microorganisms, medical biologists can estimate the likelihood of a particular microorganism causing infection, the rate at which an infection will likely spread through a population, the capacity of a pathogen to invade and damage the host, the severity and impact of an infection on individual hosts and entire populations, and the mortality rate of an infection. However, a complete understanding of virulence in many pathogens remains elusive because of the evolution of pathogens, both outside and inside the human body, and because of the many factors involved in virulence.

Virulence Factors

Virulence factors refer to the properties and mechanisms that enable a microorganism to enter a host and cause harm. Virulence factors operate at the molecular and genetic levels. A single pathogen may have one or many virulence factors, and there are huge variations in virulence among pathogens. Some bacteria species, for example, engender different diseases by combining different virulence factors. Some of the more common, and easy-to-understand, virulence factors are discussed here.

Adherence. To cause infection, pathogens must first adhere to certain cells on the surface of tissue in the host. If they do not adhere, they will be flushed away by mucus and other fluids that naturally rid the body of foreign invaders. Bacteria, in particular, have evolved mechanisms that allow them to attach themselves to host cells. Dental plaque, for example, is caused by bacteria with the power to stick fast to the teeth and gums.

Colonization. Bacteria and viruses are most harmful when they work cooperatively, or colonize, as all bacteria or all viruses, in huge numbers called colonies. These colonies provide benefit among members, but they harm the host.

Adaptation. Some pathogens produce specific enzymes (proteins that speed up chemical changes) in response to their environment that enable them to flourish and produce infection. Some enzymes cut into cells, allowing the pathogen to enter. Some dissolve the glue between cells, allowing the pathogen to spread. Others protect the pathogen from the body’s natural defenses. For example, the fungus Cryptococcus gatti (which causes a deadly but rare lung disease) develops a thick outer coating after it enters the lungs. Also, some bacteria can produce enzymes to counter the effects of antibiotics; this ability of bacteria is known as antibiotic resistance.

Toxicity. Some pathogens manufacture toxins (poisons) inside the body that immobilize, damage, or destroy vital components or functions of the host, allowing the pathogen to thrive. The agents that cause cholera, botulism, anthrax, and tetanus are examples of this type of pathogen. Toxins produced by bacteria outside the body cause many of the diseases commonly known as food poisoning, including Salmonella infection and Escherichia coli infection.

Virulence factors in pathogens and the virulence of infections are parts of the equation only. The final part involves factors within the host.

Host Factors

Virulence involves a complex interaction of the pathogen, infection, and host. Various factors inherent in or acquired by the host influence the effects of a pathogen or infection. Host factors include age, gender, genetic makeup, nutritional status, immune system status, and acquired immunity.

The status of the immune system is particularly important. Persons (hosts) with weakened immune systems, such as those with cancer or human immunodeficiency virus (HIV) infection; the morbidly obese; and drug addicts, are susceptible to microorganisms not normally harmful to healthy people. People with weakened immune systems also contract more infectious diseases, experience more severe symptoms, have more chronic infections, and heal more slowly from injuries and wounds than do healthy people.

Acquired immunity is powerful protection against infection. Humans acquire immunity, or resistance, to some pathogens and infections through previous exposure to them. That exposure can come from contracting an infectious disease or through acquired immunological resistance (vaccination). After acquiring immunity, the immune system becomes stimulated when encountering certain pathogens, and it can reduce the virulence of an infection to such low levels that no symptoms appear.


Virulence from both pathogens and infections has affected the course of civilizations. The great plague epidemics in Europe in the Middle Ages and the smallpox epidemics that decimated Native American peoples are two examples of the power of virulence.

On the positive side, the systematic study of virulence has contributed to advances in the biological sciences and to a better understanding of pathogens and infections. Researchers have unraveled the genomes of many pathogens, and based on an understanding of the makeup and evolution of pathogen virulence, medical biologists are finding ways to manage virulence by selecting for mild strains of infection, thus forcing the more virulent strains into extinction. Also, specific antigens (substances that stimulate the immune response) are being created for active immunity against some of the most dangerous pathogens.


Dieckmann, Ulf, et al., eds. Adaptive Dynamics of Infectious Diseases: In Pursuit of Virulence Management. New York: Cambridge University Press, 2005. An introductory text for infectious disease researchers.

Ewald, Paul W. “The Evolution of Virulence and Emerging Diseases.” Journal of Urban Health 75 (1998) 480-491. Provides insights into the evolution of virulence.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. This text outlines many common bacteria and describes their natural history, pathogenicity, and other characteristics.

Myers, Judith H., and Lorne E. Rothman. “Virulence and Transmission of Infectious Diseases in Humans and Insects: Evolutionary and Demographic Patterns.” Trends in Ecology and Evolution 10 (1995): 194-198. A discussion of the evolution of diseases.

Perlman, R. L. “Life Histories of Pathogen Populations.” International Journal of Infectious Diseases 13 (2009): 121-124. Argues that the virulence of pathogens should be rethought as a feature of pathogen populations, or groups, rather than as a feature of individual microorganisms within these populations.

Wilson, Michael, Brian Henderson, and Rod McNab. Bacterial Disease Mechanisms: An Introduction to Cellular Microbiology. New York: Cambridge University Press, 2002. Based on research advances in microbiology, molecular biology, and cell biology, this work describe the interactions between bacteria and human cells both in health and during infection.