Head lice intermingle as a community, and one louse may spend its day on several heads. The same person can catch lice repeatedly. If not dealt with, head lice affect the carrier adversely. When feeding, the louse pierces the scalp with its mouth-parts, pumping in saliva and drawing out a mixture of saliva and blood. While humans are not born with the ability to react to such bites, repeated contact with the sensitizing allergens contained in the saliva creates an allergic reaction, so that each further bite evokes an itchy reaction.
Wounds produced in the scalp by scratching in response to itches can become inflamed and infected.
About five thousand eggs are laid each month, and the head eventually becomes grey with drifts of empty egg shells. Such a population, when present for some months, is enough to produce the symptoms of pediculosis capitis. The child becomes itchy, tired, dull and sullen. The bright child becomes average, and the average child stupid. The life cycle of the head louse falls into three distinct phases, egg, nymph and mature louse.