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Pathogenic microbes are microbes that are pathogens and thus cause infectious diseases. This article is dedicated to human pathogenic microbes.

The organisms involved include pathogenic bacteria, causing diseases such as plague, tuberculosis and anthrax; protozoa, causing diseases such as malaria, sleeping sickness and toxoplasmosis; and also fungi causing diseases such as ringworm, candidiasis or histoplasmosis. However, other diseases such as influenza, yellow fever or AIDS are caused by pathogenic viruses, which are not living organisms and are not therefore microorganisms. As of 2007, no clear examples of archaean pathogens are known,[1] although a relationship has been proposed between the presence of some methanogens and human periodontal disease.[2]



Although the vast majority of bacteria are harmless or beneficial, a few bacteria are pathogenic. The most common bacterial disease is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis and leprosy.


Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola, AIDS, avian influenza and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between Human Herpesvirus Six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. There is current controversy over whether the borna virus, previously thought of as causing neurological diseases in horses, could be responsible for psychiatric illnesses in humans.[3]

Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which causes cold sores, to remain in a dormant state within the human body. This is called latency[4] and is a characteristic of the herpes viruses including the Epstein-Barr virus, which causes glandular fever, and the Varicella zoster virus, which causes chicken pox. Latent chickenpox infections return in later life as the disease called shingles.

Some viruses can cause life-long or chronic infections, where the viruses continue to replicate in the body despite the hosts' defense mechanisms.[5] This is common in Hepatitis B virus and Hepatitis C Virus infections. People chronically infected with the Hepatitis B virus are known as carriers who serve as reservoirs of infectious virus. In some populations, with a high proportion of carriers, the disease is said to be endemic.[6] When diagnosing Hepatitis B virus infections, it is important to distinguish between acute and chronic infections.[7]


Other pathogenic microbes include parasites and fungi and actinomycetes.


  1. ^ Eckburg P, Lepp P, Relman D (2003). "Archaea and their potential role in human disease". Infect Immun 71 (2): 591–6. doi:10.1128/IAI.71.2.591-596.2003. PMID 12540534.  
  2. ^ Lepp P, Brinig M, Ouverney C, Palm K, Armitage G, Relman D (2004). "Methanogenic Archaea and human periodontal disease". Proc Natl Acad Sci USA 101 (16): 6176–81. doi:10.1073/pnas.0308766101. PMID 15067114.  
  3. ^ Chen C, Chiu Y, Wei F, Koong F, Liu H, Shaw C, Hwu H, Hsiao K (1999). "High seroprevalence of Borna virus infection in schizophrenic patients, family members and mental health workers in Taiwan". Mol Psychiatry 4 (1): 33–8. doi:10.1038/ PMID 10089006.  
  4. ^ Margolis TP, Elfman FL, Leib D, Pakpour N, Apakupakul K, Imai Y, Voytek C. Spontaneous reactivation of herpes simplex virus type 1 in latently infected murine sensory Ganglia.J Virol. 2007 Oct;81(20):11069-74. Epub 2007 Aug 8.
  5. ^ Bertoletti A, Gehring A (2007). "Immune response and tolerance during chronic hepatitis B virus infection". Hepatol. Res. 37 Suppl 3: S331–8. doi:10.1111/j.1872-034X.2007.00221.x. PMID 17931183.  
  6. ^ Nguyen VT, McLaws ML, Dore GJ (2007). "Highly endemic hepatitis B infection in rural Vietnam". Journal of Gastroenterology and Hepatology 22: 2093. doi:10.1111/j.1440-1746.2007.05010.x. PMID 17645465.  
  7. ^ Rodrigues C, Deshmukh M, Jacob T, Nukala R, Menon S, Mehta A (2001). "Significance of HBV DNA by PCR over serological markers of HBV in acute and chronic patients". Indian journal of medical microbiology 19 (3): 141–4. PMID 17664817.  


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