Pathogen

A pathogen or infectious agent is a biological agent that causes disease or illness to its host. The term 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. The term pathogen is derived from the Greek pa?????e?a, "birth of pain."

There are several substrates and pathways whereby pathogens can invade a host; the principal pathways different episodic time frames, but soil contamination has the longest or most persistent potential for harboring a pathogen.

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, human immunodeficiency virus (HIV), or antibiotics being taken to kill other pathogens), pathogenic bacteria that were being held at bay can proliferate and cause harm to the host. Such cases are called opportunistic infections.

Some pathogens (such as the bacterium 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.