What Is an STD?

STD stands for Sexually Transmitted Disease. STDs are infections passed from one person to another through sexual contact — vaginal, anal, or oral sex. Some STDs can also be transmitted through non-sexual means such as blood or from mother to infant during childbirth. The term STI (Sexually Transmitted Infection) is often used interchangeably; technically an infection becomes a disease when symptoms appear, but many people carry STIs without symptoms.

Common STDs

• Chlamydia — bacterial; often asymptomatic. Can cause pelvic inflammatory disease (PID) if untreated. Curable with antibiotics.
• Gonorrhea — bacterial; symptoms include discharge and painful urination. Increasing antibiotic resistance is a concern. Treatable.
• Syphilis — bacterial; progresses through stages (primary sore, rash, latent, late-stage organ damage). Treatable with penicillin.
• HPV (Human Papillomavirus) — viral; most sexually active people contract at least one strain. Some strains cause genital warts; others cause cervical, anal, and oropharyngeal cancers. Vaccine available.
• Herpes (HSV-1 and HSV-2) — viral; causes oral or genital sores. Not curable but manageable with antiviral medication.
• HIV — viral; attacks the immune system. Not curable but manageable with antiretroviral therapy (ART). PrEP available for prevention.
• Trichomoniasis — parasitic; causes itching, discharge, and discomfort. Treatable with antibiotics.

STD vs STI: What's the Difference?

An STI is the presence of a pathogen (bacteria, virus, or parasite) in the body — you can have an STI without any symptoms. An STD develops when that infection causes signs or symptoms. Because most people refer to all sexually transmitted conditions as "STDs" in everyday language, both terms are widely used. The CDC now predominantly uses "STI" to reduce stigma and emphasize that many infections are asymptomatic and treatable.

How Common Are STDs?

According to the CDC, there are approximately 26 million new STIs in the United States each year. About half occur among young people aged 15–24. Chlamydia is the most commonly reported bacterial STD. Many cases go undiagnosed because screening is not routine for those without symptoms.