Understanding the life cycle of head lice is an important step in knowing how to treat lice infestations, and what your expectations should be depending on the type of lice treatment you use.
Head lice start as eggs (also called lice nits) that are glued to a hair shaft about a quarter inch (~0.5 cm) from the scalp (view lice egg pictures). Because of their protective shells, eggs are quite resistant to many lice products that use pesticides, essential oils or suffocation as a means of killing the lice. This is why lice shampoos and similar products often require additional treatments a week or so later, to treat the new lice that hatch from the unaffected eggs.
Lice eggs are attached well enough to the hair that they can withstand fingers, hair washing, brushes and everyday activities; however, if used properly a lice comb can be an effective way of removing lice eggs.
Eight or nine days into the lice life cycle, a nymph will emerge from the egg. Nymphs are very small and like to stay close to the scalp where they feed. Nymphs will spend 9-12 days growing into adult lice, at which stage they can reproduce.
An adult louse is about the size of a sesame seed (see head lice pictures). They can be found traveling farther along the hair than nymphs, which means they are more susceptible to falling off the hair, or climbing onto another head through hair-to-hair contact.
Adults live up to 30 days on their host. Females lay roughly four eggs a day, or roughly 88 eggs during their lifetime. People get infested with head lice when a female louse transfers to their head and starts laying eggs.