Head lice are spread by direct contact with the hair of someone who is infected.[6] The cause of head lice infestations are not related to cleanliness.[5] Other animals, such as cats and dogs, do not play a role in transmission.[6] Head lice feed only on human blood and are only able to survive on human head hair.[4][5] When adults, they are about 2 to 3 mm long.[8] When not attached to a human, they are unable to live beyond three days.[5] Humans can also become infected with two other lice – the body louse and the crab louse. To make the diagnosis, live lice must be found.[5] Using a comb can help with detection.[5] Empty eggshells (known as nits) are not sufficient for the diagnosis.[5]