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Adult head lice are approximately the size of a sesame seed [2.5 mm (0.1 in.) to 3.5 mm (0.14 in.)] and can vary in color from white to brown to dark gray.

Louse eggs (nits) are much smaller [1 mm (0.03 in.)] and look like light gray or brown oval cylinders glued to individual hair shafts.

Head lice spread easily from one person to another through close bodily contact or through shared clothing or personal items such as hats or hair brushes.