Pubic lice are usually spread through sexual contact. Rarely, infestation can be spread through contact with an infested person's bed linens, towels, or clothes. A common misunderstanding is that infestation can be spread by sitting on a toilet seat. This isn't likely, since lice cannot live long away from a warm human body. Also, lice do not have feet designed to walk or hold onto smooth surfaces such as toilet seats. Infection in a young child or teenager may indicate sexual activity or sexual abuse.

Adult crab lice are only a little over half the size of body or head lice, rarely more than 1/12" long; their last two pairs of legs terminate in hooked mitts that resemble crab claws. These lice are confined to coarse pubic hair and sometimes armpits, eyebrows and eyelashes. Pubic lice move very little in the pubic region and produce few eggs. The most common method of transmission of crab or public lice is by sexual intercourse. When infested pubic hair detaches, lice can hatch on underwear, towels, in beds, or on toilet fixtures. If their immediate environment is above 50o F., a pair of pubic lice could infest another person without personal contact.