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Relapsing fever, as the name implies, is characterized by recurrent acute episodes of fever. These are followed by periods of defervescence of increasing duration. The infection is caused by various spirochete species of the Borrelia genus. Spirochetes are a unique species of bacteria that also cause syphilis, Lyme disease, and leptospirosis. The fever relapses result from spirochetal antigenic variation. Relapsing fever, if untreated, may be fatal.

Relapsing fever is an arthropod-borne infection spread by lice (Pediculus humanus) and ticks (Ornithodoros species). Two main forms of this infection exist: tickborne relapsing fever (TBRF) and louse-borne relapsing fever (LBRF).

TBRF is caused by 8 or more Borrelia species (eg, Borrelia hermsii, Borrelia turicatae, Borrelia parkeri,Borrelia duttonii), while LBRF is caused solely by Borrelia recurrentis.

TBRF and LBRF vary significantly in terms of epidemiology. A soft-bodied tick (Ornithodoros) transmits multiple Borrelia species that cause endemic relapsing fever, whereas the human body louse transmits B recurrentis, which causes an epidemic form of relapsing fever. Unlike hard ticks, Ornithodoros adult ticks are able to live for many years, feed repeatedly on blood meals, lay eggs, and perpetuate their life cycle. In addition, Ornithodoros ticks may survive long periods in a fasting state. In fact, Ornithodoros turicata ticks have been known to transmit spirochetes in the laboratory setting after 7 years without a blood meal.