Borrelia hermsii is a spirochete bacterium representing the endemic causative agent of tick-borne relapsing fever in eastern regions of North America (more specifically, the eastern United States, and British Columbia region of Canada).

It is spread by the soft-bodied tick Ornithodoros hermsi.

Human infections characteristically occur among campers and people temporarily lodging in wooden accommodations in proximity to rodents.

Human disease is usually relatively mild with low fever.

[1] B. hermsii is endemic to regions of the U.S. with high elevation (whereas B. turicatae is endemic to low-lying regions such as Texas and Florida).