Head lice. The mere mention triggers an itch response. Pediculus capitis and its close relative, pubic lice, Pthirus pubis, are human-specific parasites, and have lived with us for thousands of years. They cannot survive anywhere but in human hair and cannot eat anything other than human blood. Body, or clothing, lice are a different species, and are known vectors for several human diseases, including louse-borne typhus fever.
Hans Zinsser, an HMS professor of bacteriology and immunology from 1923 until his death in 1940, authored Rats, Lice and History, a biography of typhus. Although Zinsser’s thesis that the head louse was a vector for typhus was later disproved, his research on insect vectors advanced the understanding of human infectious disease.
But even the lowly louse does some good in the world. Researchers are studying the divergent evolution of head and body lice to determine when humans shed most of their body hair and began wearing clothing. Losing body hair allowed early humans to sweat more efficiently, useful when running distances to hunt food, and the protection provided by clothing allowed migration out of Africa into cooler climes.