Syphilis Bacterium. Treponema Pallidum Subsp. Pallidum On Cultures Of Cotton Tail Rabbit Epithelium Cells Sf1Ep. Treponema Pallidum Is The Causative Agent Of Syphilis. In The United States Over 35 600 Cases Of Syphilis Were Reported By Health Officials In 1999. (Photo By BSIP/UIG Via Getty Images)