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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 colour from white to brown to dark grey.

Louse eggs (nits) are much smaller [1 mm (0.03 in.)] and look like light grey 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.