Never ignore genital sores or blisters, unusual discharge, burning urination, pelvic or testicular pain, rectal symptoms after anal sex, or a palms-and-soles rash. But the bigger rule: most STIs are silent, so test after any risk, symptoms or not.
Chlamydia is asymptomatic in most women; herpes and HPV spread from people who feel fine. So the actionable rule is testing after new partners, unprotected sex, a partner's diagnosis, or any symptom above, plus routine screening with multiple partners. Most bacterial STIs (chlamydia, gonorrhea, syphilis, trichomoniasis) are curable with antibiotics; HIV and herpes are manageable. Ignoring costs more than testing, especially for fertility: untreated chlamydia and gonorrhea cause pelvic inflammatory disease and infertility in women. One masquerader: sudden severe testicular pain can be torsion, not infection, and is a same-hour emergency. Pregnant women should treat any symptom as urgent, since several STIs transmit to the baby.
What to do
Common questions
What STD symptoms should I never ignore?
The symptoms that should never be ignored: genital sores, blisters, or warts (herpes, syphilis, HPV); unusual discharge from the penis or vagina, especially yellow, green, gray, or foul-smelling; burning or pain with urination; pelvic or lower abdominal pain in women (possible pelvic inflammatory disease, which threatens fertility); testicular pain or swelling in men; rectal pain, discharge, or bleeding after anal sex; and a rash on the palms and soles or a painless sore followed by body rash (secondary syphilis, which is resurging). Also on the list: painful or swollen lymph nodes in the groin, sores in the mouth or throat after oral sex, and any symptom in a pregnant woman, because several STIs pass to the baby. Here is the part that reframes everything, per the CDC: most STIs cause no symptoms at all, or symptoms so mild they get missed; chlamydia is famously silent in most women, and HPV and herpes spread from people who feel perfectly fine. That is why "no symptoms" is not "no STI": the real rule is to test after any new partner, any unprotected sex, any partner's diagnosis, or any symptom above, and regularly if you have multiple partners. Testing is straightforward (urine, swab, or blood), most STIs are curable with antibiotics (chlamydia, gonorrhea, syphilis, trichomoniasis), and the rest (HIV, herpes) are manageable, so the cost of ignoring is almost always higher than the cost of testing. And the emergency edge: testicular pain that starts suddenly and severely can be torsion, not an STI, and is a same-hour ER problem.