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'''Pathogenic microbes''' are [[microbe]]s that cause [[infectious disease]]s. This article is dedicated to human pathogenic microbes.

The organisms involved include [[pathogenic bacteria]], causing diseases such as [[bubonic plague|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,<ref>{{cite journal |author=Eckburg P, Lepp P, Relman D |title=Archaea and their potential role in human disease |journal=Infect Immun |volume=71 |issue=2 |pages=591-6 |year=2003 |id=PMID 12540534}}</ref> although a relationship has been proposed between the presence of some methanogens and human [[periodontal disease]].<ref>{{cite journal |author=Lepp P, Brinig M, Ouverney C, Palm K, Armitage G, Relman D |title=Methanogenic Archaea and human periodontal disease | url=http://www.pnas.org/cgi/content/full/101/16/6176 | journal=Proc Natl Acad Sci U S A |volume=101 |issue=16 |pages=6176-81 |year=2004 |id=PMID 15067114}}</ref>

==Bacteria==
{{Main|Pathogenic bacteria}}
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]]. 

==Viruses==
{{See|Viruses#Viruses and disease|Table of clinically important viruses}}
Examples of common human diseases caused by viruses include the [[common cold]], [[influenza|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 [[neurology|neurological]] diseases in horses, could be responsible for [[psychiatry|psychiatric]] illnesses in humans.<ref name=Chen_1999>{{cite journal
 |author=Chen C, Chiu Y, Wei F, Koong F, Liu H, Shaw C, Hwu H, Hsiao K
 |title=High seroprevalence of Borna virus infection in schizophrenic patients, family members and mental health workers in Taiwan
 |journal=Mol Psychiatry
 |volume=4
 |issue=1
 |pages=33-8
 |year=1999
 |pmid=10089006
}}</ref>

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 organism]]s, 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 cause cold sores, to remain in a dormant state within the human body. This is called latency<ref>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.</ref> 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 (medical)|chronic]] infections, where the viruses continue to replicate in the body despite the hosts' defense mechanisms.<ref name="pmid17931183">{{cite journal
 |author=Bertoletti A, Gehring A
 |title=Immune response and tolerance during chronic hepatitis B virus infection
 |journal=Hepatol. Res.
 |volume=37 Suppl 3
 |issue=
 |pages=S331–8
 |year=2007
 |pmid=17931183
 |doi=10.1111/j.1872-034X.2007.00221.x
}}</ref> 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 (epidemiology)|endemic]].<ref name="pmid17645465">{{cite journal
|author=Nguyen VT, McLaws ML, Dore GJ
|title=Highly endemic hepatitis B infection in rural Vietnam
|journal=
|volume=
|issue=
|pages=
|year=2007
|pmid=17645465
|doi=10.1111/j.1440-1746.2007.05010.x}}</ref> When diagnosing Hepatitis B virus infections, it is important to distinguish between [[Acute (medical)|acute]] and [[Chronic (medical)|chronic]] infections.<ref name="pmid17664817">{{cite journal
|author=Rodrigues C, Deshmukh M, Jacob T, Nukala R, Menon S, Mehta A
|title=Significance of HBV DNA by PCR over serological markers of HBV in acute and chronic patients
|journal=Indian journal of medical microbiology
|volume=19
|issue=3
|pages=141–4
|year=2001
|pmid=17664817
|doi=}}</ref>

==Other==
Other pathogenic microbes include [[parasites]] and [[fungi]].

==References==
{{reflist|2}}

{{Pathogenic microbes}}

[[Category:Microbiology]]
[[Category:Pathogenic microbes]]