STD (sexually transmitted disease), also known as STI (sexually transmitted infection), refers to an infection passed from one person to another primarily through sexual contact, including vaginal, anal, or oral sex, and sometimes via blood, semen, vaginal fluids, or skin-to-skin contact. Caused by bacteria (e.g., chlamydia, gonorrhea, syphilis), viruses (e.g., HIV, herpes simplex, human papillomavirus/HPV, hepatitis B), or parasites (e.g., trichomoniasis), many are asymptomatic or mild initially but can lead to complications like infertility, chronic pain, organ damage, increased HIV risk, or cancer if untreated. Common examples include chlamydia (often silent genital infection), gonorrhea (discharge or burning urination), syphilis (progressive stages with sores, rash, and later systemic effects), genital herpes (recurrent painful blisters/sores), HPV (warts or asymptomatic, linked to cancers), and HIV (immune system destruction leading to AIDS). Transmission occurs during intimate contact with an infected partner, and some can pass from mother to child during pregnancy or birth.