Epidemic typhus results from infection by Rickettsia prowazekii, a Gram negative, obligate intracellular bacterium. At least two strains can be distinguished by genetic analysis. One strain is found only in humans; the other also occurs in flying squirrels in the United States.
Iowa State University
Iowa State University Center for Food Security and Public Health, "Typhus Fever – Rickettsia prowazekii" (2004). Center for Food Security and Public Health Technical Factsheets. 141.