Relapsing fever is an infection caused by bacteria in the Borrelia family. There are two major forms. Tick-borne relapsing fever (TBRF) is transmitted by the Ornithodoros tick and occurs in Africa, Spain, Saudi Arabia, Asia, and certain areas in the western US and Canada. Louse-borne relapsing fever (LBRF), which is transmitted by body lice, is prevalent in the developing world (Asia, Africa, and Central and South America).

Within two weeks of infection, affected people develop sudden-onset high fever. In louse-borne relapsing fever, the initial episode usually lasts three to six days and is usually followed by a single, milder episode. In tick-borne relapsing fever, multiple episodes of fever occur and each may last up to three days. Individuals may be free of fever for up to two weeks prior to a relapse into fever.

In both forms, the fever episode may end in "crisis," which consists of shaking chills, followed by intense sweating, falling temperature, and low blood pressure -- this stage may result in death in up to 10% of individuals.

After several cycles of fever, some people may develop dramatic central nervous system signs such as seizures, stupor, and coma. The Borrelia organism may also invade heart and liver tissues, causing inflammation of the heart muscle (myocarditis) and inflammation of the liver (hepatitis). Diffuse bleeding and pneumonia are other complications of this illness.

In the United States, TBRF usually occurs west of the Mississippi River, particularly in the mountainous West and the high deserts and plains of the Southwest. In the mountains of California, Utah, Arizona, New Mexico, Colorado, Oregon, Washington, infections are usually caused by B. hermsii and are often acquired in cabins in forests. It is possible that the risk to humans of relapsing fever agents extends now into the southeastern United States.

LBRF is mostly a disease of the developing world and it often occurs in epidemics. It is currently prevalent in Ethiopia and Sudan. Famine, war, and the movements and congregations of refugees often result in epidemics of LBRF. The largest epidemics of LBRF during this century occurred during the two world wars. Millions of people were infected in these epidemics, with at least one million deaths.