A parasite is any living organism, larger than a fungus, a bacterium, or a virus, that lives in or on humans and feeds on them. Parasitic diseases have been an extremely serious problem for humans throughout recorded history, and some of them, such as malaria, are causing serious infections in many more people than only 30 years ago.
Malaria kills an estimated million children every year in Africa alone.
Parasites can live on or in the skin, in any of the body tissues, in the intestines, or in the vagina of women. The most common skin parasites are insects and include the various forms of lice and scabies. These are spread by close, not necessarily sexual, contact with other infected persons. Head lice are common among school-age children and are also seen in people who are debilitated or living in crowded conditions with poor bathing facilities. In the U.S., lice usually do not carry other diseases, but historically lice-borne infections, such as typhus, have caused major epidemics.