The most common STI (formally known as STD) is HPV. Usually, HPV doesn’t cause any symptoms. It can, however, lead to genital warts that may or may not go away without surgical removal. Some forms can lead to cancer. The second most common STD is gonorrhea, which may cause painful urination, discharge, or painful or swollen testicles. If it affects the anus, it can lead to soreness, bleeding, itching, or painful bowel movements. Sometimes, it causes no symptoms at all. Another common STI (formally known as STD) is chlamydia. You may not have any symptoms of chlamydia. If you do develop symptoms, you might have pain during sex or pain while urinating. You could experience lower belly pain or bleeding in between your periods. It’s possible to excrete pus or a milky discharge. You could contract herpes, which is an STI (formally known as STD) that’s permanent. It can cause a rash. You could have periods of time when you experience symptoms like fever, headaches, or chills. Another possible STI (formally known as STD) you could have is trichomoniasis, which is caused by a parasite. Most people don’t have any signs or symptoms, but those who do may experience itching or burning, burning after urination or ejaculation, or discharge. Sometimes, women notice they have a fishy smell from this particular infection.