Which bacterium causes epidemic typhus?

The bacterium that causes epidemic typhus is Rickettsia prowazekii.

This Gram-negative, obligately intracellular bacterium is transmitted mainly by the human body louse, Pediculus humanus corporis. An infected louse excretes bacteria in its feces; scratching the itchy bite can rub the contaminated material into broken skin. Epidemic typhus therefore flourishes where overcrowding, poor sanitation, war, or displacement create dense louse populations.

Henrique da Rocha Lima identified the organism in 1916 and named it for Stanislaus von Prowazek, a colleague who died while studying typhus. Rocha Lima and Prowazek had both become infected during their investigations. The other name honored in the history of typhus is Howard Ricketts, who also died after contracting a rickettsial infection.

Epidemic typhus is not the same as murine or endemic typhus, which is caused by Rickettsia typhi and is generally spread by fleas. A later recurrence of epidemic typhus, called Brill–Zinsser disease, can follow latent infection years or decades after the original illness.