Genital warts are a sexually transmitted infection caused by the human papilloma virus.  Chlamydia is a common sexually transmitted disease , caused by bacteria called Chlamydia trachomatis. Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. Trichomoniasis is a very common STD caused by infection with Trichomonas vaginalis , a protozoan parasite.