Typhus is a disease caused by a group of bacteria called Rickettsia. Three forms of typhus are recognized: epidemic typhus, a serious disease that is fatal if not treated promptly; rat-flea or endemic typhus, a milder form of the disease; and scrub typhus, another fatal form. The Rickettsia species of bacteria that cause all three forms of typhus are transmitted by insects. The bacteria that cause epidemic typhus, for instance, are transmitted by the human body louse; the bacteria that cause endemic typhus are transmitted by the Oriental rat flea; and bacteria causing scrub typhus are transmitted by chiggers.

Typhus takes its name from the Greek word typhos meaning smoke, a description of the mental state of infected persons. Typhus is marked by a severe stupor and delirium, as well as headache, chills, and fever. A rash appears within four to seven days after the onset of the...