Pathogenic bacteria

Why it's important

Bacteria that cause disease are called pathogenic bacteria. Bacteria can cause diseases in humans, in other animals, and also in plants. Some bacteria can only make one particular host ill; others cause trouble in a number of hosts, depending on the host specificity of the bacteria. The diseases caused by bacteria are almost as diverse as the bugs themselves and include food poisoning (Source: Public Health Dept. Vic, Au), tooth ache (Source: Cosmetic Dentistry), anthrax (Source: Nebraska Health & human Services System), even certain forms of cancer. It is impossible to sum up all bacterial diseases and it would be pretty boring. The Infectious Diseases fact sheets (Source: New York State Department of Health) gives brief descriptions of diseases, including infectious diseases. Some diseases are named after the organisms that cause them, or is it the other way round?

If you want to have a look at pathogenic bacteria under the microscope:

Some pathogenic bacteria have received disproportionate attention in the press, e.g. the 'flesh-eating bacteria' (Source: WebMD symtomchecker) , which in real life are called Streptococci. Indeed they can cause spectacular, but fortunately uncommon symptoms. In the press, pathogenic bacteria are sometimes represented as (deadly) dangerous enemies that lurk in the dark, unseen, ready to attack you. Although that is exaggerated some bacteria can be life treatening, for example

Legionella pneumoniae

, the causative of Legionnaire's disease. These bacteria survive in moist places like air conditioners or hot-water pipes. Though potentially life treatening, bacterial infections do not kill all their victims. If they did, and if they spread fast, they would eradicate their hosts completely. Although a popular theme for thrillers, this is a 'mission impossible' in real life. Some bacteria kill a high percentage of people infected (they have a so-called a high mortality rate), but their relatively inefficient rate of spreading makes up for that.


When an infectious disease spreads around an area, and the cases of new infections reach a certain number, we call it an epidemic. Read about epidemics (Source: American Natl History Museum) or visit the Epidemics Home Site (National Library of Medicine). If, on the other hand, a certain disease is always present in low numbers of cases in a given area, that disease is said to be endemic in that area. Some epidemics become wide-spread and quickly reach distant parts of the world: in modern times people travel fast and frequent, and our bacteria travel with us. A classical example of such a pandemic (in this case caused by a virus) was a new type of influenza ("Spanish 'flu") that reached continent after continent early this century, killing thousands of people on it's way. Although only a small proportion of the infected people died (the virus had a low mortality) so many people got infected that even the small proportion of deaths amounted to large numbers. An epidemic or pandemic can only occur if the population is not immune to that disease. Read our special feature file on the history of infectious diseases about the times that pathogens could spread unlimited. Our exhibit on our immune system explains how immunity can prevent disease. So why do epidemics occur? Either because they are caused by diseases that did not exist before, like AIDS, caused by the HIV virus, or because new variants of bacteria (or viruses) arrive in an area where they were not endemic before. This is why epidemics of the common 'flu' occur frequently: the virus causing influenza is able to change itself sufficiently to bypass our immunity built up from prior infections. Although not caused by bacteria, this site on influenza is very interesting if you want to understand epidemics of infectious diseases (Source: Ultranet EJ Kimball). Whenever a new virus type arises, there is the potential for a new epidemic. Fortunately, most of these new types are not as vicious as the Spanish 'flu. The experts are currently keeping an eye on bird flu, a strain of influenza that has not (yet?) learned to jump from person-to-person (Source: CDC). If that would happen, a new pandemic could be the result.

An example of a bacterial disease that caused successive epidemics, and even pandemics, in recent times is Vibrio cholerae, the cause of cholera. Epidemiology is the study to establish the cause of a disease.

  • Read this classic of cholera in London in the last century: an epidemiological 'whodunit' with John Snow as the chief inspector (Source: Ultranet EJ Kimball).
  • Another classic epidemic in history was small pox raging through Europe in the 14th Century. Read our special feature file on the Black Death.

Bacteria have invented many different strategies to make us ill. These strategies, called bacterial pathogenicity, are the subject of an important division of medical microbiology. Understanding how certain bacteria make us ill can result in better treatment, vaccination, or prevention of that infectious disease. An example of such research is how treatment of ear infections can be improved (Source: Montana State Uni). In another exhibit some of the common mechanisms of bacterial pathogenicity are explained. In order to keep this information balanced, now that you know what pathogenic bacteria are, why not also check out how commensal bacteria are good for you.

The content of this page is copyright of the Virtual Museum of Bacteria and was written by Dr. T. M. Wassenaar (curator), with support of the Foundation for Bacteriology.

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Page last modified: 08 Mar 2007


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Pathogenic bacteria
Bacteria that cause disease are called pathogenic bacteria. Bacteria can cause diseases in humans, in other animals, and also in plants. Some bacteria can only make one particular host ill; others cause trouble in a number of hosts, depending on the host specificity of the bacteria.
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