Typhus is a group of febrile illnesses caused by rickettsial organisms. Epidemic (or louse-borne), murine, and scrub typhus are the more common types. Murine typhus, also called endemic typhus, is a mild form, and epidemic typhus is the most severe. The rare cases of epidemic typhus that have been reported in the United States have been related to exposure to flying squirrels. Epidemic typhus, also known as jail or prison fever, camp fever, famine fever, and hospital fever, has been found in areas of natural disaster, war, or poverty, where sanitation is difficult to maintain. Epidemic typhus occurs more often in colder months, when clothes are not laundered as frequently.

Worldwide, murine typhus is the most common type of typhus. In the United States, it occurs mostly in southern California during the summer and fall and in southern Texas during the spring and summer. Scrub typhus has not been reported in the United States but is endemic in northern Japan, Southeast Asia, and eastern Australia.