Head-louse infestation or head lice (also referred to colloquially in British, Irish, and Australian English as nits) is a human medical condition caused by the colonization of the hair and skin by the parasitic insect Pediculus humanus capitis—the head louse. Typically, only the head or scalp of the host is infested, although the disease can occur in other hairy parts of the body, like leg hairs. Head lice feed on human blood (hematophagy), and itching from louse bites is a common symptom of this condition. Treatment typically includes application of topical insecticides such as a pyrethrin or permethrin, although a variety of folk remedies are also common.
Lice infestation in general is known as pediculosis, and occurs in many mammalian and bird species. The term pediculosis capitis, or simply "pediculosis", is sometimes used to refer to the specific human pediculosis due to P. humanus capitis (i.e., head-louse infestation). Humans are hosts for two other lice as well—the body louse and the crab louse.
Head-louse infestation is widely endemic, especially in children. It is a cause of some concern in public health, although, unlike human body lice, head lice are not carriers of other infectious diseases.