The only way that a person can become infected with head lice is through prolonged, head-to-head contact with an infected person. Therefore, head lice are typically acquired by children from someone they know well, such as a family member or close friend. It is extremely unlikely that head lice will be transmitted by sharing clothing or personal items, such as combs or pillows. Most experts agree that lice that fall off the head are probably ill or dying and are therefore unlikely to reproduce.

The main symptom of a head lice infection is an itchy scalp. This is not the result of being bitten, but is actually an allergic reaction. However, not everybody is allergic to head lice, therefore itching does not always develop and the lice may go undetected. Even if a person is allergic to head lice, it can take up to three months from infection for the itching to begin. In rare cases, a rash may appear on the back of the neck due to a reaction to lice droppings.

Even when the head is closely inspected, head lice can be easy to miss. It can be difficult to distinguish whether eggs and nits are alive or dead. Nits can also remain glued to hairs, even after successful treatment. Therefore, unhatched eggs or nits alone are not enough to confirm an active head lice infection. An accurate diagnosis depends on a live louse being found via a reliable method, such as detection combing. Parents may be unaware of how to look for head lice effectively, so it's important that pharmacy teams are able to offer help and advice.