Although there are many types of lice, the head louse, or Pediculus humanus capitis, is a parasitic insect that can be found on the head and, more rarely, the eyebrows and eyelashes of people. Head lice (the plural form of louse) feed on human blood several times a day and live close to the scalp to maintain their body temperature.

Head lice come in three stages: egg, nymph and adult (learn how big lice are in their various stages). Eggs are often called lice nits, although many people refer to nits as the empty egg shells left behind by a hatched louse.

A head louse has six claws that allow it to crawl around from hair strand to hair strand. It can move from one head to another this way, which is why head-to-head contact is the primary way head lice are spread between people.

A head-lice infestation occurs when a female adult louse makes it onto a new head and lays eggs. When those eggs hatch, the lice will most likely stay on that head throughout the entire lice life cycle. Unless the head is treated (see pediculosis treatment) and all lice and eggs are killed, the lice infestation will continue for however long the lice can live.