Tick-borne relapsing fever
Relapsing fever is caused by the bacteria Borrelia. In the United States it typically occurs in mountainous areas of the western states. It is transmitted to humans by the bites of argasid (soft) ticks that become infected when feeding on infected rodents such as squirrels and chipmunks. These ticks are found where rodents burrow and nest, often in older buildings. The ticks typically feed only at night and, unlike the ticks that cause Lyme disease, do not remain attached for prolonged times. They can survive for long periods between blood meals, and typically do not leave a noticeable bite wound. In the western United States and British Columbia, exposure commonly occurs in older buildings and cabins located in higher elevations.