Studies show quite a variation in the number of people affected by head lice at any one time and, according to the PHMEG report, there is evidence that head lice are becoming increasingly prevalent. Two separate studies among Welsh schoolchildren found head lice rates of 4.1 per cent and 8.3 per cent respectively. In a separate survey of 14 English schools, the proportion of pupils affected ranged from four to 22 per cent. In addition, a 2003 study from one district health authority found that two per cent of children were affected at the time of the survey, but that 37.4 per cent reported an infection at some time in the previous year. Head lice are most common in children aged four to 11, however all ages could potentially become infected.

The following factors are thought to increase the risk of developing an infection:

• Being younger than 12 years old
• Being female €“ this may be due to the way girls interact with their family and friends
• Coming from a family with four or more children
• Belonging to a lower income family.

A head lice infection is not the result of dirty hair or poor hygiene. In fact, there is no evidence to suggest that head lice have a preference for either clean or dirty hair €“ they will infect all types of hair, regardless of its condition or length. Head lice can only infect humans and cannot be passed on to, or caught from, animals. They are also different to other types of human lice, including body or pubic lice (crabs).