Human lice (Pediculus humanus) are world-wide, obligate ectoparasites that infest humans; the medical term condition of being infested with lice is pediculosis. This species is comprised of two morphologically identical, interbreeding (at least in laboratory conditions, although not in natural ones), but behaviorally distinguishable subspecies that occupy non-overlapping habitats: Pediculus humanus corporis (human body lice, also called Pediculus humanus humanus) and Pediculus humanus capitus (human head lice). These subspecies are thought to have diverged about 110,000 years ago when humans started wearing clothing.

As their name suggests, head lice attach their eggs at the base of hairs, and spend their entire life cycle on the head of their host. Head lice, unlike body lice, are not known to be disease vectors, but are common and persistent nuisance of millions of people in the US alone, mainly school aged children. Lice are wingless and do not hop or jump, head lice spread by crawling from hair to hair. Thus close head-to-head contact between children or children and adults, including sharing hair brushes and combs is the chief means of infestation.

(Morgan 2001; Smith; Wikipedia 2011a, 2011b)