Most lice are scavengers, feeding on skin and other debris found on the host's body, but some species feed on skin secretions and blood. Most are found only on specific types of animals, and, in some cases, only to a particular part of the body; some animals are known to host up to fifteen different species, although one to three is typical for mammals, and two to six for birds. Lice generally cannot survive for long if removed from their host.

• A louse's (singular for lice) color varies from pale beige to dark gray; however, if feeding on blood, it may become considerably darker.
• Female lice are usually more common than the males, and some species are even known to reproduce asexually.
• A louse's egg is commonly called a nit; living lice eggs tend to be pale white while dead lice eggs are more yellow.