Head lice infest people of all ages but are especially common on preschool and elementary school aged children, including their household members and caretakers.
Head lice are not a sign of poor hygiene and all socioeconomic groups are affected. The adult head louse measures 2-4 mm long and is adapted to life on human hair and the scalp, less commonly lice can also be found on eye brows and eye lashes. Head lice infest the areas around the head and neck and attach eggs (called nits) approximately 4-6 mm from the base of the hair shaft. Lice and nymphs are generally a dull yellow or tan to grayish-white in color, but will appear to be gray or almost black when full of blood. A louse may appear darker in a person with dark hair. The female louse attaches her eggs (nits) to hair shafts close to the scalp with a waterproof, glue-like substance. Nits are frequently pale yellow or white color, but may also become dark if the embryo dies, or will be transparent after the louse emerges. Live nits may also appear to be the same color of the hair of the infested individual. These nits appear as evidence of infestation. Itching (pruritis) is a manifestation of the feeding process and is the primary symptom caused by an allergic reaction to the louse bites.
People will often not experience symptoms; therefore itching should not be relied upon as a definitive diagnostic sign. The first time an individual is infested with lice, it may take up to 4 to 6 weeks before possible itching symptoms develop. Other signs and symptoms of an infestation may include: a tickling feeling in the hair or a sensation of something moving, sleeplessness or irritability, and/or sores in the head caused by scratching (the sores can become infected with bacteria). A thorough examination of the head will reveal the telltale yellowish to white nits. Head lice do not transmit diseases.
Getting head lice is not related to cleanliness of the person or his or her environment. Hair length, the frequency of shampooing or brushing does not influence the risk of a head lice infestation.
Head lice are mainly spread by direct contact with the hair of an infested person. The most common way to get head lice is by head-to-head contact with a person who already has head lice. Such contact can be common among children during play at school, home, and elsewhere (e.g. sports activities, playgrounds, camp, and slumber parties).
Source: The Ohio Department of Health