Forgotten diseases: relapsing fever.

Relapsing fever is a group of conditions that is characterized by recurrent febrile episodes interspaced with periods of no symptoms at all. They are arthropod-borne spirochetal infections of the Borrellia genus and occur in two main forms: tick-borne and louse-borne. Tick-borne relapsing fever is a zoonosis that is found worldwide. The louse-borne relapsing fever is caused by B. recurrentis, and is seen in the developing world. This form usually occurs in the setting of overcrowding. It is spread from person to person and can occur in epidemics, including large ones involving millions of people. Mortality rate may reach 40% when untreated, depending on host factors such as nutritional status and concurrent illness.