Leptospira, Borrelia, andSpirillum cause disease characterized by clinical stages with remissions and exacerbations.Leptospira organisms are very thin, tightly coiled, obligate aerobic spirochetes characterized by a unique flexuous type of motility. The genus is divided into two species: the pathogenic leptospiresL interrogans and the free-living leptospireL biflexa. Serotypes ofL interrogans are the agents of leptospirosis, a zoonotic disease. The primary hosts for this disease are wild and domestic animals, and the disease is a major cause of economic loss in the meat and dairy industry. Humans are accidental hosts in whom this disseminated disease varies in severity from subclinical to fatal. The first human case of leptospirosis was described in 1886 as a severe icteric illness and was referred to as Weil's disease; however, most human cases of leptospirosis are nonicteric and are not life-threatening. Recovery usually follows the appearance of a specific antibody.

In contrast to the pathogenic leptospires, serotypes ofL biflexa exist in water and soil as free-living organisms. AlthoughL biflexa has been isolated from mammalian hosts on occasion, no pathology has been found, and it does not infect experimental animals. Because of the widespread distribution ofL biflexa in fresh water and the capability of leptospires to pass through 0.45 to 0.22-μm-pore-size sterilizing filters, they have been found as contaminants of filter-sterilized media.

Borrelia species are responsible for the relapsing fevers and Lyme disease. The organisms are transmitted to humans primarily by lice or ticks. Relapsing fevers are acute recurrent illnesses characterized by febrile episodes that recede spontaneously but generally reappear with decreasing intensity and duration.Borrelia recurrentis is responsible for the louse-borne or epidemic type of relapsing fever with humans serving as the reservoir host. The disease does not occur in the United States. In the western United States and CanadaB hermsii andB turicatae are the most frequent causes of tick-borne or endemic type of relapsing fever, withB hermsii responsible for most human cases. Rodents are the primary reservoir for these borreliae. Lyme disease is another tick-borne illness and is caused byB burgdorferi. The disease occurs in the north temperate zone. The majority of cases in the United States occur in the north central and northeastern states and California. Rodents are the major reservoir for this spirochete. Antibodies play an important role in immunity to borrelial infections.

A single member of the genusSpirillum, S minum, is pathogenic for humans.Spirillum minum causes one type of rat bite fever, which is characterized by recurrent fever. The pathogenesis of the organism is obscure, but the host can produce a spirillicidal antibody.