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Human lice (Pediculus humanus) are world-wide, obligate, wingless ectoparasites that spend their full life cycle on humans; the medical term for the 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; this subspecies is also known as Pediculus humanus humanus) and Pediculus humanus capitis (human head lice). As their names suggest, body lice are found hiding in and attaching their eggs to clothing, whereas head lice infest the host's scalp, and attach their eggs to the base of hairs. These subspecies are thought to have diverged about 110,000 years ago when humans started wearing clothing. Body lice infest mostly those living with poor hygiene and who do not have access to bathing facilities and clean sheets and clothes, as infestations do not persist through bathing and laundering. Body lice can carry such diseases as louse-borne typhus (Rickettsia prowazeki), trench fever (Rochalimaea quintana), and louse-borne relapsing fever (Borrellia recurrentis). Head lice, on the other hand, are not known to be disease vectors, but are a common and persistent nuisance of millions of people in the US alone, mainly school aged children.
(Morgan 2001; Smith; Wikipedia 2011a, 2011b)