If you catch one or two lice, they may breed and increase slowly in number. At this stage, most people don’t have any symptoms and won’t know they have lice unless they look very carefully for them.

For the first two or three months, there is usually no itch, but then the scalp may start to itch badly. This is due to an allergy, not to the louse bites themselves.

Most people only realise they have head lice when this itch starts. By then they’ve had lice on their head for two or three months without knowing it.

In most infections, there aren’t more than a dozen or so lice on the scalp at any one time.

Some people never get the itch, including adults. They may have a few lice on their heads for years without knowing it, and can pass them to other people.

Louse droppings may fall on to the pillow during the night. Pillows may then get dirty more quickly than usual.