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Influenza viruses are common pathogens. This image has a magnification of about 100.000 times

A pathogen is an infectious thing, such as a virus, bacteria, fungi or parasite, which causes a disease. This ability is called Pathogenicity.

The body contains many natural orders of defense against some of the common pathogens (such as Pneumocystis) in the form of the human immune system and by some "helpful" bacteria present in the human body's normal flora. However, if the immune system or "good" bacteria is damaged in any way (for example, chemotherapy, HIV, or antibiotics being taken to kill other pathogens), pathogenic bacteria that were being held at bay can grow and cause harm to the host. Such cases are opportunistic infections.

Some pathogens (such as the bacterium Yersinia pestis which may have caused the Black Plague, the Variola virus, and the malarial protozoa) have been responsible for very big numbers of deaths and injuries.

The most famous and lethal outbreak was the 1918 Spanish flu pandemic (type A influenza, H1N1 subtype), which lasted from 1918 to 1919. It is not known exactly how many it killed, but estimates range from 20 to 100 million people.[1][2] This pandemic has been described as "the greatest medical holocaust in history" and may have killed as many people as the Black Death.[3]

The huge death toll was caused by an extremely high infection rate of up to 50% and the extreme severity of the symptoms. One observer wrote, "One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and skin also occurred".[1] The majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive bleeding and oedema in the lungs.[4]

In plants, fungi are the main cause of infectious disease.


  1. 1.0 1.1 Knobler S, Mack A, Mahmoud A, Lemon S, ed. "1: The Story of Influenza". The Threat of Pandemic Influenza: Are We Ready? Workshop Summary (2005). Washington, D.C.: The National Academies Press. pp. 60–61.
  2. Patterson, KD; Pyle GF (Spring 1991). "The geography and mortality of the 1918 influenza pandemic". Bull Hist Med. 65 (1): 4–21. PMID 2021692 .
  3. Potter CW (October 2001). "A History of Influenza". Journal of Applied Microbiology 91 (4): 572–579. doi:10.1046/j.1365-2672.2001.01492.x . PMID 11576290 .
  4. Taubenberger, J; Reid A, Janczewski T, Fanning T (December 29 2001). "Integrating historical, clinical and molecular genetic data in order to explain the origin and virulence of the 1918 Spanish influenza virus". Philos Trans R Soc Lond B Biol Sci 356 (1416): 1829–39. doi:10.1098/rstb.2001.1020 . PMC 1088558 . PMID 11779381 .