Lice (singular louse) are an order of small flightless insects. There are about 3,000 different species of lice. The vast majority pose no threat to human health and some are even beneficial, feeding on loose skin and other waste. Most lice are species specific.
However, three species of lice are of serious concern to humans - head lice, body lice and pubic lice. They can be vectors for serious diseases, such as typhus. In addition, they can result in infestations which, although medically harmless, can cause itching and other skin irritation.
Lice are a far more serious threat to the health of birds, where they can cause severe skin problems and can transmit disease.