Head lice are very small insects, which live on blood, which they suck from the scalp. They attach their small, white, shiny eggs (nits) to shafts of hair. Each egg is the size of a flake of dandruff. Lice are spread from person to person by direct contact and also by sharing brushes and combs. Epidemics of lice at schools are common, especially in girls with long hair.

Head lice usually present as itching and scratching of the scalp. Many small red spots can be seen on the scalp. While it is difficult to see the lice, their eggs are easy to recognise as they are firmly attached to the hairs. Unlike dandruff, nits are not easy to remove. Rarely lice may also infest other parts of the body. Some children have head lice without itching or scratching and only the nits are noticed. All children with head lice can spread the lice to others.

As with scabies, the scratching can cause secondary bacterial infection (impetigo) which must also be treated.