Head lice are small insects with six legs. They are often said to be "as large as a match head"; in fact, they are often not much bigger than a pin head, and rarely bigger than a sesame seed (the seeds on burger buns).

They live on, or very close to the scalp, and don’t wander far down the hair shafts for very long.

The louse’s mouth is like a very small needle. It sticks this into the scalp and drinks the blood.

They can only live on human beings; you can’t catch them from animals.

Nits are not the same thing as lice. Lice are the insects which move around the head. Nits are egg cases laid by lice, stuck on to hair shafts; they are smaller than a pin head and are pearly white.

If you have nits it doesn’t always mean that you have head lice. When you have got rid of all the lice, the nits will stay stuck to the hair until it grows out.

You only have head lice if you can find a living, moving louse (not a nit) on the scalp.