HSV is most frequently harmless...that's why doctors don't test for it unless you ask.
Syphilis: severe problems with the heart, brain, and nerves that can result in paralysis, blindness, dementia, deafness, impotence, and even death if it's not treated.
Gonorrhea: serious and permanent problems in both women and men including pelvic inflammatory disease, infertility, increased risk of ectopic pregnancy, epididymitis, can also affect the prostate and can lead to scarring inside the urethra, making urination difficult. Gonorrhea can spread to the blood or joints. This condition can be life-threatening.
Chylamydia: cervicitis, urethritis, endometritis, inflammation of the Bartholin glands, PID, abscess, Infertility, Fitz-Hugh–Curtis syndrome, miscarriage, preterm labor, complications in newborns, premature delivery, conjunctivitis), infection of the nose and throat, pneumonia, ear infection, urethritis
HIV: pneumocysis pneumonia, cytomegalovirus, TB, dementia, wasting syndrome, non-hodgkin's lymphoma, kaposi's sarcoma, lipodystrophy
Herpes: itching, blisters, headache, backaches, flu like symptoms (often mistaken for yeast infections, bladder infections, bacterial infections), pregnancy complication