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PathogenA pathogen or infectious agent is a biological agentthat causes diseaseor illnessto its host. The term is most often used for agents that disrupt the normal physiologyof a multicellularanimal or plant. However, pathogens can infect unicellular organismsfrom all of the biological kingdoms. The term pathogen is derived from the Greekπαθογένεια, "birth of pain." The human body has many natural defenses against some of the more common pathogens (such as Pneumocystis) in the form of the human immune systemand 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 (such as by chemotherapy, human immunodeficiency virus(HIV), or antibioticsbeing taken to kill other pathogens), pathogenic bacteria that was being held at bay can proliferate and cause harm to the host. Such cases are called opportunistic infections. Some pathogens (such as Yersinia pestis, which may have caused the Black Plague, and the Variola virus) have been found to be responsible for massive amounts of casualties and have had numerous effects on afflicted groups. Plague is carried by fleas. Of particular note in modern times is HIV, which is known to have infected several million humans globally, as well as Severe Acute Respiratory Syndrome(SARS). Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of vaccination, antibiotics and fungicide, pathogens continue to threaten human life. Types of pathogensBelow is a listing of different types of notable pathogens as categorized by their structural characteristics, and some of their known effects on infected hosts.
Transmission of pathogens
See also
de:Krankheitserreger fr:Pathogène nl:Pathogeen pl:Patogen zh:?? sv:Patogen Categories: Microbiology| Pathology
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