of human diseases during the 20th century, head louse infestation remains stubbornly prevalent. in 1997, 80% of american elementary schools reported at least one outbreak of lice. lice infestation during that same period was more prevalent than chickenpox. about 6–12 million children between the ages of 3 and 11 are treated annually for head lice in the united states alone. high levels of louse infestations have also been reported from all over the world, including israel, denmark, sweden, u.k., france, and australia. the united kingdom's national health service report that lice have no preference for any type of hair be it clean, dirty, or short. the number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g. school), and socioeconomic status were found to be factors in head louse infestation in iran. other studies found no relationship between frequency of brushing or shampooing. the california department of public health indicates that chronic head lice infestation may be a sign of socioeconomic or family problems. children between 4 and 13 years of age are the most frequently infested group. in the u.s., african-american children have lower rates of infestation. head lice (pediculus humanus capitis) infestation is most frequent on children aged 3–10 and their families. females get head lice twice as often as males, and infestation in persons of afro-caribbean or other black descent could be rare due to difference in hair shape or width. but these children may have nits that hatch and the live lice could be transferred by head contact to other children. stigma head lice infestations are notably common, as is the stigma associated with those who experience infestations. such stigma is even evidenced in the english language as the term "lousy", an adjective that describes something as very poor, bad, or disgusting. misperceptions of those infected with head lice include that it is associated with low socioeconomic status, poor hygiene, unhealthiness, immigration status, and homelessness. though these negative beliefs are unfounded, they can lead to consequences for both the caregivers and the affected individual, such as social exclusion and isolation from peers, victim-blaming, caregiver strain, inappropriate or unsafe treatment practices, and missed work or school. public-health implications over-treatment or mismanagement of head lice, which can be driven by stigma, has important implications at the level of the individual and community. though evidence-based guidelines from the cdc, american academy of pediatrics (aap) and national association of school nurses (nasn) all recommend discontinuing "no-nit" policies in schools (meaning that a child does not need to be free of nits before returning to school), 80 percent of schools in the united states still maintain stringent policies that prevent children with infestations from attending. thus, to foster a return to school in a timely fashion, these policies can encourage unsafe or harsh treatment practices, including chemicals like bleach or kerosene. similarly, over-treatment of head-lice using pesticide-based pediculicides has been linked to increased resistance and declining efficacy of these treatments. society and culture to a louse (on a lady's bonnet). perhaps the most widely known cultural reference to pediculosis capitis, occurring in a noted poem by robert burns. other animals lice infestation in general is known as pediculosis, and occurs in many mammalian and bird species. lice infesting other host species are not the same organism as that which causes head lice infestations in humans, nor do the three louse species which infest humans infest any other host species.