Head lice are tiny wingless insects that are grey-brown in colour. They are the size of a pinhead when they hatch and 3mm long (the size of a sesame seed) when fully grown.
Head lice cannot fly, jump or swim. They are spread by head-to-head contact and climb from the hair of an infected person to the hair of someone else.
A female head louse lays eggs by cementing them to hairs (often close to the root) where they will be kept warm by the scalp. The eggs are pinhead size and difficult to see.
When the baby lice hatch 7 to 10 days later, the empty eggshells (nits) remain glued in place. Nits glisten white and become more noticeable as the growth of the hair carries them away from the scalp.
Head lice feed by biting the scalp and sucking blood through it. They take 6 to 10 days to become fully grown. Once mature, a head louse can transfer from head to head.
After mating, a female may start to lay eggs as early as the seventh day after she has hatched. So to break the life cycle and stop head lice spreading, they need to be removed from the head before the sixth day after hatching.
Children are most commonly affected by head lice, although anyone with hair can catch them.
Children are often affected by head lice because they tend to have more head-to-head contact while at school or during play. Head lice are most common in children between 4 to 11 years old.
Head lice can be effectively treated using medicated lotions or by wet combing, using a specially designed head lice comb.
You can also ask your school nurse, health visitor, GP or pharmacist if you are worried about head lice or you want more advice about how to treat them.