Sexually transmitted diseases (STDs) are infections transmitted through sexual activity like vaginal, anal, or oral sex. STDs can affect both sexes and are caused by bacteria, viruses, parasites, or fungi. Common STDs include chlamydia, gonorrhea, HPV, herpes, syphilis, and HIV/AIDS. Symptoms of STDs vary but may include abnormal discharge, pain during urination or sex, genital itching or sores, swollen lymph nodes, and flu-like symptoms. STDs can be prevented through safe sex practices like condom use and getting routine testing.