Lice are parasitic upon birds and mammals. They feed on the blood of their hosts and incidentally transmit disease causing organisms.
As permanent ectoparasites of most birds and mammals, lice they exhibit a remarkable level of host specificity. Most individuals will spend their entire life cycle on a single host, with transmission from one host to another largely occurring opportunistically during close contact. Specializations in the lifestyle and diet of lice determine their ecological niche and major taxonomic divisions. They can be broadly separated into those that feed on skin debris, feathers and fur, and those that have specialized in blood feeding.
To date there are more than 3000 known species of lice and yet many more remain undescribed. With the possible exception of those species that impinge on the activity of humans and their livestock, the true biology of this cryptic group of insects remains obscure.