A pathogen (literally "birth of pain" from the Greek παθογένεια) is a biological agent that can cause disease to its host. A synonym of pathogen is "infectious agent". The term "pathogen" is most often used for agents that disrupt the normal physiology of a multicellular animal or plant. However, pathogens can infect unicellular organisms from all of the biological kingdoms (see Viruses, below).
The human body has many natural defenses against some of the more 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 (such as by chemotherapy or HIV, or by antibiotics being 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.
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 life on Earth, and can be particularly devastating to its human inhabitants.
Threat of pathogens
Some pathogens (such as Yersinia pestis, which caused the Black Plague, and the aforementioned Variola virus) have been found responsible for massive amounts of casualties and have had numerous effects on afflicted groups. Of particular note in modern times is HIV/AIDS, which is known to have infected several million humans globally, threatening entire populations in certain parts of the world, as well as the recently emerged Severe Acute Respiratory Syndrome, or SARS.
Below is a listing of different types of notable pathogens as categorized by their structural characteristics, and some of their known effects on infected hosts.