Humans contract lice by sharing brushes, hats, close head-to-head
contact, etc. The scalp louse deposits nits on the hair next to
the scalp, and scalp hair grows 0.5 mm daily (thus, the presence
of nits 15 cm from the scalp indicates that the infestation is approximately
9-month-old). New viable eggs have a creamy-yellow color; empty
eggshells are white. The infestation first manifests as severe pruritus
of the back and sides of scalp. Crusts and secondarily impetiginized
lesions are common and may extend onto neck, forehead, face, and
ears. In extreme cases, the scalp can become a mass of matted hair,
lice, nits, and purulent discharge called plica polonica.