Epidemic typhus is caused by R. prowazekii and is transmitted by the human body louse. Eastern flying squirrels and their lice and fleas maintain R. prowazekii in a zoonotic cycle.

• The louse lives in clothing under poor hygienic conditions, particularly in colder climates and classically at times of war or natural disaster.
• Lice feed on pts with epidemic typhus and then defecate the organism on their subsequent host during their next meal. The pt autoinoculates the organism while scratching.
• Brill-Zinsser disease is a recrudescent form of epidemic typhus whose occurrence years after acute illness suggests that R. prowazekii remains dormant in the host, with reactivation when immunity wanes.