Three distinct varieties of lice are specifically parasitic for humans (figure 1) . Two of them, Pediculus humanus capitis (the head louse) and Pediculus humanus humanus (the body louse), are closely related variants of the same species, despite their different habits. The third is Phthirus pubis, the pubic louse commonly known as the "crab." The clinical manifestations, diagnosis, and treatment of pediculosis corporis and pediculosis pubis are discussed separately. (See "Pediculosis corporis" and "Pediculosis pubis and pediculosis ciliaris".)
The head louse is an adaptable creature that is found worldwide, and infestations occur in individuals of all socioeconomic backgrounds . Children are affected most commonly; with the exception of the common cold, pediculosis capitis affects a greater number of elementary school students in North America than all other communicable diseases combined. It has been estimated that in 1997 approximately one of every four elementary level students in the United States was infested [3,4].
Males appear to be less susceptible to lice infestations than females [5-7], and in the United States, black children are affected much less frequently than whites and others . The reasons for these findings are uncertain. It is possible that the lower incidence in black children in the US may be related to the prevalence of lice in the US that are less capable of grasping the shape or width of certain types of hair . Studies investigating the impact of long hair length on the risk of louse infestation have yielded conflicting results [5,6,9].
Although direct contact with the head of an infested person generally is considered the primary mode of transmission of pediculosis capitis [8,10], data are insufficient for definitive conclusions on the significance of specific routes of louse transmission. In particular, the degree to which inanimate objects contribute to the spread of pediculosis capitis is controversial [11,12]. While a study performed in an experimental setting suggested that use of hair dryers, combs, or towels could contribute to louse transmission , other studies investigating the prevalence of lice on fomites hats, bedding, and floors after exposure to an infested individual have not supported these routes as important modes of louse transmission [14-16]. Additional studies are necessary to clarify the role of inanimate objects in the risk for louse transmission.
The head louse is a gray-white, mobile insect 3 to 4 mm in length (picture 1). The female is a little longer than the male. Both sexes are equipped with mouth parts adapted to sucking blood and legs adapted to grasping hairs. The life span of the female is about one month, during which time she lays 7 to 10 eggs each day, cementing them firmly to the base of a host hair. The eggs, commonly called "nits," are oval capsules that hatch in eight days, releasing nymphs that require another eight days to mature (picture 2A-B). After hatching, egg cases become white and more visible. Adults feed voraciously both on the scalp and adjacent areas of the face and neck.