Syphilis is caused by a thin, tightly coiled spirochete, Treponema pallidum subspecies pallidum (119). It is microaerophilic and cannot grow on standard culture media. It is a member of the family Spirochaeticea and is related to other spirochete genera which have the capacity to infect man, namely Borrelia and Leptospira. Other pathogenic treponemes for man include T. pallidum subspecies pertenue, the causative agent of yaws; T. carateum, the causative agent of pinta; and T. pallidum subspecies endemicum, the agent associated with non-venereal or endemic syphilis.