Lice

Lice are tiny wingless insects that feed on human blood every 3 to 6 hours. Lice inject their own saliva when they bite, which often causes an allergic reaction in the human host. The allergic reaction makes the bites itch.

Head lice

Head lice are 3 mm (0.12 in.) to 4 mm (0.16 in.) long, about the size of a sesame seed. Head lice lay their eggs (nits) on head hair. The eggs are tightly attached to hairs by a gluelike substance made by female lice. The eggs hatch in about 6 to 10 days and require another 7 to 13 days before they mature into adult, egg-laying lice. The nits of head lice may live for up to 2 weeks attached to hairs that have fallen from a person's head. The most common symptom is itching, especially on the scalp, which may develop weeks or even months after lice infest the person.

Head lice may be spread through close personal contact, shared personal items (combs, brushes, hats, helmets, clothing, or earphones), or shared bedding.

Lice cannot survive long without human body contact because they must feed on blood to live. Head lice can live for 1 to 3 days without body contact.

Pubic lice ("crabs")

Pubic lice are about 1 mm (0.04 in.) to 2 mm (0.08 in.) in size. Pubic lice lay their eggs pubic hair but also may lay eggs in the area around the anus, armpits, body and facial hair, and eyelashes. The most common symptom of pubic lice is itching of the affected areas.

Pubic lice are spread mainly through sexual contact and are very contagious (most people become infected after a single exposure to another infected person). But the lice and eggs may survive long enough on personal items such as clothing or towels to be spread to another person. Up to one-third of people who have pubic lice also have some other sexually transmitted disease.

Lice cannot survive long without human body contact because they must feed on blood to live. Pubic lice can live about 2 days without body contact. Pubic lice eggs may also survive on clothing and towels for up to 10 days.

Body lice

Body lice are 3 mm (0.12 in.) to 4 mm (0.16 in.) long. The lice and eggs are generally not seen on the skin but may be found in the seams of the person's clothing. Intense itching, especially at night, is the main symptom of body lice. Itchy sores from body lice occur in body areas such as the armpits, waist, and trunk where seams of clothes press against the skin.

Body lice are most often spread by contact with personal items, especially clothing and hats. They occasionally may be spread by direct personal contact.

Lice cannot survive long without human body contact because they must feed on blood to live. Body lice, which live in clothing, not on the body itself, can live without human blood for 7 to 10 days.

Under certain conditions, such as may occur during natural disasters or war, body lice may transmit life-threatening diseases such as typhus, relapsing fever, and trench fever.

Most lice infestations do not cause long-term health problems, but they can cause severe itching and discomfort and may be embarrassing to some people. Body lice may transmit other diseases.