Lice spend their entire life as ectoparasites on humans and, unlike other insects, they have a relatively consistent environment. Their close contact with human skin ensures favorable temperatures of 82 to 86 F, and an abundant food supply. Lice do not abandon their hosts unless the body temperature substantially changes due to death or high fever. Once dislodged from a person's body or clothing, they will infest a new human host in the immediate proximity. If a new host is not found within one to eight days, the lice will starve to death.
Lice infestations are common throughout the world.
In the United States alone an estimated ten million cases of pediculosis occurred during 1985. Lice are spread through sharing contaminated clothing, hats, scarves, combs, hair brushes, and other headgear, or as a result of close physical contact with an infested person. Other occasional sources are bedding, furniture, rugs, and floor surfaces where dislodged lice may be present. Once infested, an individual usually carries a few dozen lice. However, some people have been known to carry several hundred lice, and on rare occasions, one to two thousand lice.