Gonorrhea can cause a rash if left untreated (disseminated gonorrhea). Syphilis can cause a rash in the second stage. Reiter's syndrome, which sometimes follows chlamydia, can cause a rash. Trichomoniasis can cause a rash in the genital area due to irritation from the infection. While not the kind of red eruption we think of as a "rash," warts, molluscum, and herpes can be skin problems caused by STDs.