Nits are tiny insects, that can be up to 3mm long.
They feed on human blood several times a day, and live close to the human scalp, according to the Centers for Disease Control and Prevention.
They’re easily spread by personal contact – particularly by sharing belongings, or head-to-head contact.
The only way to know if someone has a head lice infestation is to look for live lice, or their eggs.
Nits can create a sensation of something scuttling across your head, according to medical website Healthline.
The feeling can make people itch their head, caused by the saliva and faeces of the lice.
“The most common symptom of any type of lice is itching,” it said. “Lice bites cause an allergic reaction that causes this itchy feeling.
“However, you may not feel itchy right away, especially if it’s a light infestation. You may not notice any symptoms for up to six weeks the first time you get lice.”
Other common signs of head lice include irritability and difficulty sleeping, said Healthline.
Finding red bumps on your head, shoulders or pubic area are also symptoms of nits.
You could also spot a head lice infestation by the visible appearance of sores from scratching. If you find small, white objects in your hair, it could also be signs of nits.
The British Association of Dermatologists said: “Head lice have to feed on human blood several times a day to survive, and their bites, saliva and faeces often make the scalp itchy.”
Head lice are contagious, and sharing personal belongings could increase your risk of infestation, said the NHS.
Washing clothes and sheets regularly could help to prevent spreading any nits, while vacuuming the floor and furniture may also provide protection against the parasites.
You can buy medication to get rid of nits. Some shampoos contain ingredients to treat an infestation.
See your GP if you’re unsure if you have an infestation.