Adult head lice are roughly 2-3 mm long. 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. Dogs, cats, and other pets do not play a role in the transmission of human lice.
Both over-the-counter and prescription medications are available for treatment of head lice infestations.
Image: Two lice viewed under an electron microscope. Note the claws used to grasp onto individual hairs.