Which bacterium causes epidemic typhus, historically spread by the human body louse?

Rickettsia prowazekii causes epidemic typhus, historically spread by the human body louse.

The bacterium is an obligate intracellular pathogen, meaning it must live and reproduce inside host cells. In epidemic typhus, infected body-lice feces can contaminate scratched skin or mucous membranes. Crowded conditions, limited access to washing facilities, and clothing infestation historically allowed outbreaks to spread rapidly.

Epidemic typhus is distinct from murine, or endemic, typhus, which is mainly associated with Rickettsia typhi and fleas. It is also different from typhoid fever, which is caused by Salmonella Typhi and is transmitted through contaminated food or water. R. prowazekii is named partly in honor of microbiologists Stanislaus von Prowazek and Henrique da Rocha-Lima. Antibiotic treatment is effective, but untreated epidemic typhus can be severe or fatal.