UNIT 4 PELVIC INFECTIONS. Sexually Transmitted Diseases: Syphilis is a sexually transmitted disease.The causative organism of syphilis is the spirochete Treponoma palladium. This disease usually starts with syphilitic lesions in the genital tract and occurs mainly in the genitals, rectum and mouth. The disease is caused by direct contact with other persons who have primary or secondary syphilitic lesions. Its incubation period is 9-90 days. These symptoms are mainly seen