Lice range in size from one to four millimeters long and are wingless with a flattened body. Their mouthparts are adapted for piercing and sucking. Their short legs have claws suitable for grasping and clinging to skin hairs and fibers. The tiny louse cannot jump, hop or fly, but can move relatively quickly.

The adult and the nymph stage of development immediately prior to adult are both blood sucking. They introduce saliva into their host during feeding. This causes a small reddened, raised and hard area (erythematous papule) that appears within a few hours.

When infestation initially occurs, it is generally mild, with 3 to 10 lice present. Each female will lay eggs (nits) at the rate of 3-10 per day and live 30-40 days. The lice are completely dependent on blood from their hosts and usually cannot survive away from hosts for more than 24 to 48 hours. The louse usually sucks blood for long periods of time, becomes engorged, and appears reddish in color.