infectious disease characterized by recurring episodes of fever separated by periods of relative well-being and caused by spirochetes (spirochete), or spiral-shaped bacteria, of the genus Borrelia. The spirochetes are transmitted from one person to another by lice (louse) (genus Pediculus) and from animals to humans by ticks (genus Ornithodoros). The tick-borne disease is frequently contracted by persons visiting wooded campsites or cabins. The louse-borne disease spreads under conditions of crowding, cold weather, and poor hygiene, all of which favour the spread of lice. Epidemics of the disease have occurred during wars, earthquakes, famines, and floods.

After the spirochete has lived about one week in its newly infected host, the person experiences a sudden onset of high fever, chills, headache, and muscle aches. The symptoms persist for about a week in cases contracted from lice and usually for a shorter period in the tick-borne disease. The attack ends in a crisis of profuse sweating, low blood pressure, low temperature, and malaise, after which the patient is fairly well until, about a week later, febrile symptoms return. Additional relapses may follow—rarely more than one or two in the louse-borne disease but up to 12 (usually decreasing in severity) in cases contracted from ticks. The mortality is variable, ranging from nil in some tick-transmitted varieties to 6 percent or as high as 30 percent in some louse-borne epidemics associated with famine conditions. The spirochetes may invade the central nervous system and cause a variety of usually mild neurological symptoms. An enlarged liver or spleen, rashes, and inflammation of the eye and heart also may be noted in patients with relapsing fever.

Borrelia spirochetes were the first microbes to be associated clearly with serious human disease. German bacteriologist Otto Obermeier observed the organisms in the blood of relapsing-fever patients in 1867–68 and published his observations in 1873. They are easily seen in dark-field microscopic preparations of the patient's blood collected during the height of the febrile attack, but they disappear from the blood during the intervals between attacks. These observations, as well as the relapsing symptoms, have been related to changes in the antigenic characteristics of the spirochetes. As the patient develops immunity to the prevailing type and recovers from the attack, a new (mutant) type of the spirochete develops and produces the relapse. Because neither the bite nor the excreta of the louse is infectious, human infections usually result from crushing the louse on the skin while scratching.

A diagnosis of relapsing fever can be made by specific serological tests or by identification of the loosely coiled organism in the patient's blood. penicillin and other antibiotics have proved effective against the disease. Inadequate therapy commonly results in relapse after treatment, probably because of the persistence of live spirochetes in the brain, where the drug concentration does not reach curative levels. After treatment these protected spirochetes may reinvade the bloodstream. Prevention by the elimination of the vectors that transmit the disease is of the utmost importance.