Epidemic typhus (louse-borne typhus) is caused by infection with Rickettsia prowazekii, which is transmitted to humans by the body louse that lives within clothing and is found in conditions of poor hygiene. Wartime, famine, overcrowded conditions, and cold weather favor the proliferation of the louse vector. Infrequent clothing changes and bathing are factors in louse proliferation. Inhalational exposure may occur indoors where high concentrations of louse feces can aerosolize.

After a 7-day incubation period, the abrupt onset is heralded by an intense headache and fever. Chills, myalgias, and prostration are common, and a nonproductive cough may also be present. Fever rises to 39ºC-40°C (102.2°F-104°F) in an unremitting pattern. After 4-5 days of illness, a macular, papular, and/or petechial rash usually appears. Meningismus is rarely reported. Central nervous system involvement may include confusion, delirium, coma, transverse myelitis, and cranial nerve palsies. Tinnitus and deafness have been reported. Splenomegaly is common. Renal insufficiency, pneumonia, gangrene, myocarditis, and multiple organ failure may appear. Full recovery may take months.

Epidemic typhus can recur years later in previously infected patients as Brill-Zinsser disease, a milder form of the initial illness but one that can lead to infection of the vector (lice) and regenerate a previously quelled epidemic.

In the United States, an animal reservoir of R. prowazekii exists in the southern flying squirrel with reported habitat in the eastern United States and ranging west as far as the Midwest (Minnesota to eastern Texas). At least 15 human cases have been diagnosed serologically. Most cases in the United States had direct or indirect contact with flying squirrels and appeared during colder months (November to March).

The disease is also seen in the rural highlands of Central and South America, Africa (Ethiopia and Burundi in particular), and South Asia. Poor outcome is associated with poor nutritional status and older age. Mortality rate is 20% overall, but in compromised populations (eg, the elderly), it may be as high as 50%.