Head lice (scientific name Pediculus capitis) are parasitic insects that feed on human blood and cause severe itching of the head from their bites. Pediculosis is the general term used for infestation of the body or hair with adult lice, larvae, and nits (eggs). Head lice are relatively common and are not a major health hazard. More than 12 million Americans, mostly children and school professionals, are infested with lice each year. Lice is plural for louse.