Feeling lousy may mean more than having a bad day, particularly if one's head is itchy. Infestations by the human head louse plague many school districts across America and around the world. Head lice, known scientifically as Pediculus capitis, are blood-sucking members of the Class Insecta, and as small parasites, are exquisitely fit to live on the scalp and neck hairs of human hosts.

View a second image of a human head louse.

View a third image of a human head louse.

Demonstrating a near-perfect illustration of coevolution, the six legs of the head louse are adapted for efficiently grasping the person's hair shafts. Head lice infest new hosts mainly by direct head-to-head contact with infected hair, but may be transferred by contaminated combs, hats, or other hair and head accessories. For a brief period of time, human head lice can survive living freelty on upholstered furniture and bedding. Children are more frequently infested than adults and louse infestations seem to have little to do with personal hygiene, sanitation practices, or socioeconomic classes.

At least once a day, the parasitic insect must feast on a blood meal or else die. After eight days of development, a nymphal louse hatches from its egg or nit, beginning the life cycle of feeding, growing, and maturing until at age 9 to 12 days, the adult stage is reached. Each female louse may deposit in excess of 100 eggs at the rate of approximately six per day. Although the average host's head may house only a dozen or so active lice at any given time, there may be hundreds of nits visible on the scalp under magnification. Closely related body lice, as their name implies, are found on the human body everywhere except the scalp and facial hair. Although head lice are more of an annoyance than a threat, associated ill advised "cures" using caustic or toxic substances to eliminate the lice may actually create the greatest harm. In some cases, no treatment is recommended, particularly if the nits detected are nonviable.

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