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Head lice are not known to transmit any disease and therefore are not considered a health hazard.

Head lice infestations can be asymptomatic, particularly with a first infestation or when an infestation is light. Itching ("pruritus") is the most common symptom of head lice infestation and is caused by an allergic reaction to louse bites. It may take 4–6 weeks for itching to appear the first time a person has head lice.

Other symptoms may include:

  • a tickling feeling or a sensation of something moving in the hair;
  • irritability and sleeplessness; and
  • sores on the head caused by scratching. These sores caused by scratching can sometimes become infected with bacteria normally found on a person’s skin.