Head lice are wingless insects approximately the size of a sesame seed that feed on human blood. They are the most common form of lice infestation in the United States. The majority of cases occur in children between the ages of 3 and 10 and their families.

Head lice live approximately 30 days, with the female laying between five and 10 eggs per day. Nits (lice eggs) have a white to brown appearance and are usually found on hair strands near the scalp. They are attached firmly and may be difficult to remove. Nits hatch within approximately six to 10 days as nymphs, becoming adults about 10 days after that.

Infestations of head lice are spread through head-to-head contact with an infested individual or through sharing certain personal items (e.g., hats, helmets, brushes, pillows). Head lice are largely dependent on their host for survival, in most cases dying after 24 hours if separated from the scalp and unable to receive a blood meal. Intense scalp itching (pruritus) is the most common symptom. However, many people with infestations of head lice show no symptoms and are unaware of their presence. Such people may be considered "carriers." 

Treatments for head lice infestation can often be administered at home without a physician’s prescription. More serious or resistant infestations may require prescription treatments. Some common treatment methods include topical pediculicides (e.g., permethrin), wet combing and oral medication (ivermectin).

Many schools have rules forbidding the return of children to school following the discovery of head lice until the child has been initially treated. Outbreaks in schools are often controlled by group examination of teachers and students under direct light and by giving students individual wall hooks or lockers for storing hats and jackets.

If a case of head lice is identified, checking all household members for the insect is recommended to minimize the likelihood of spreading head lice to others. Habitual washing of items that may have been in contact with the head (e.g., furniture, bed sheets, combs, brushes) of someone with head lice can help prevent future infestations as well as reduce the risk of spreading them to others.