Epidemic typhus is a disease that has played an important role in history. Because typhus is transmitted by the human body louse, epidemics of this disease break out when humans are in close contact with each other under conditions in which the same clothing is worn for long periods of time. Cold climates also favor typhus epidemics, as people will be more likely to wear heavy clothing in colder conditions. Typhus seems to be a disease of war, poverty, and famine. In fact, according to one researcher, Napoleon's retreat from Moscow in the early nineteenth century was caused by a louse. During World War I, more than three million Russians died of typhus, and during the Vietnam war, sporadic epidemics killed many American soldiers.
Epidemic typhus is caused by Rickettsia prowazekii. Humans play a role in the life cycle of the bacteria. Lice become infected with the bacteria by biting an infected human; these infected lice then bite other humans. A distinguishing feature of typhus disease transmission is that the louse bite itself does not transmit the bacteria. The feces of the lice are infected with bacteria; when a person scratches a louse bite, the lice feces that have been deposited on the skin are introduced into the bloodstream.
If not treated promptly, typhus is fatal. Interestingly, a person who has had epidemic typhus can experience a relapse of the disease years after they have been cured of their infection. Called Brill-Zinsser disease, after the researchers who discovered it, the relapse is usually a milder form of typhus, which is treated with antibiotics. However, a person with Brill-Zinsser disease can infect lice, which can in turn infect other humans. Controlling Brill-Zinsser relapses is important in stopping epidemics of typhus before they start, especially in areas where lice infestation is prominent.