Head lice are wingless insects that live on the scalp and neck. They are grey or brown, have six legs and are about 2mm long when fully grown. There can be up to 12 adult head lice on an infested scalp at any one time. They may be embarrassing and occasionally uncomfortable, but head lice are not generally a causes of illness. Head lice infestation can cause an itchy head or neck, or a rash, which is often worse behind the ears or on the back of the neck. However, a person may have been infested for several months before the itchiness is noticed, and some people may not report itching at all. It's important to check anyone who has been in contact with someone with confirmed head lice so that all cases can be treated and reinfestation prevented. Other signs of head lice infection are nits stuck to the hairs as they grow out, and louse dropping, which look like a fine black powder and may be seen on pillows or sheets.