Spirochetes are gram-negative, motile, spiral bacteria, from 3 to 500 micrometres long. Spirochetes are unique in that they have endocellular flagella (axial fibrils, or axial filaments), which number between 2 and more than 200 per organism, depending upon the species. Each axial fibril attaches at an opposite end and winds around the cell body, which is enclosed by an envelope. Spirochetes are characteristically found in a liquid environment (e.g., mud and water, blood and lymph).

Treponema includes the agents of syphilis (T. pallidum) and yaws (T. pertenue). Borrelia includes several species transmitted by lice and ticks and causing relapsing fever (B. recurrentis and others) and Lyme disease (B. burgdorferi) in humans. Spirochaeta are free-living, nonpathogenic inhabitants of mud and water, usually in oxygen-free regions. leptospirosis, caused by Leptospira, is principally a disease of domestic and wild mammals and is a secondary infection of humans.