Head lice are ectoparasites. Basically, they suck up blood, glue their eggs (nits) onto a human hair out of which the nymph emerges. The nymph stage grows into an adult and the cycle repeats.
After a web search I find that head lice complete their entire life cycle on the human scalp. See for example the CDC page on lice for a credible and informative website. In fact, all websites I have found so far mention that lice live on the scalp. The wikipedia page explicitly states
Head lice [...] spend[...] their entire lives on the human scalp...
However, none of the sources I have dug up mentions explicitly that lice live only on the scalp and cannot survive anywhere else.
- Notably, I am wondering if head lice can survive in a beard, or in other hairy parts of the human body. Is it enough to eradicate them from the scalp with lice shampoo and a lice comb, or can a few covert outcast lice survive somewhere else on the body just to repopulate the scalp after the war is over?