Adult head lice are roughly 2—3 mm long. They can move on the hair and scalp, while the baby head lice only move on the scalp. Head lice infest the head and neck and attach their eggs to the base of the hair shaft. Lice move by crawling; they cannot hop or fly.
Head lice infestation, or pediculosis, is spread most commonly by close person-to-person contact. It can also be spread by sharing hats, hair brushes, anything that can go on the head. Dogs, cats, and other pets do not play a role in the transmission of human lice.