Borreliosis is a disease caused by members of the bacterial genus Borrelia. Borrelia recurrentis is the cause of louse-borne or epidemic relapsing fever of humans. Borrelia vincentii and related borreliae appear on mucous membranes, particularly of the mouth and genitals, are easily cultivated, and are transmitted by contact. Subsequent to an immune response by an infected host, a small percentage of borreliae, either circulating in the blood or residing in organs, undergo antigenic variation. The severity of borreliosis varies from epidemic to epidemic, and in endemic relapsing fever it varies from one case or group of infections to another. Nausea, vomiting, and diarrhea are common in louse-borne and African tick-borne relapsing fevers. The designation “pseudodysentery” is frequently used for such instances, particularly when the symptoms persist into the apyrexial period.