Given the right conditions of warm weather, lack of health education in schools and careless parents, one louse can travel around five heads in a day. The female headlouse Pediculus capitis lays 250 eggs in its lifetime (a few weeks), sucks its own weight in blood every 24 hours, and mates 32 times a night. The offspring have a 60% survival rate. Optimum fertility depends on a regular intake of female hormone, so lice are rarely found on post-adolescent males. The saliva of the louse is an anti-coagulant and commonly setting up an allergic reaction, causing the peron to feel "lousy". Over a long period, children with head louse infestation become anaemic and have difficulty concentrating -- they become "nitwits". Long-term lice infestation also brings constant scratching, which weakens the normal defences of the skin leading to acute and contagious skin diseases, such as impetigo, which are liable to cause serious scarring if not recognized and treated in time.
Head lice could have been wiped out years ago. Effective insecticides have been available for 20 years. Indifference, prejudice and ineptitude has go in the way. For example, home application of insecticide head shampoos just allow resistant strains of lice to develop because they are not potent enough to completely kill the eggs. Only the more concentrated lotions do that. However, unsystematic treatment of a population means that repeated infestations happen, with further need for treatment by potent insecticides (such as DDT), which poses other health hazards to the user. In addition, the laissez-faire attitude of health authorities to the problem means there is no national survey and it is hard to know if there is an overall increase in the lice population or just pockets.