Head-lice infestations are common, especially in children.[5] In Europe, they infect between 1 and 20% of different groups of people.[4] In the United States, between 6 to 12 million children are infected a year.[6] They occur more often in girls than boys.[5] It has been suggested that historically, head lice infection were beneficial, as they protected against the more dangerous body louse.[10] Infestations may cause stigmatization of the infected individual.[5]