What Is an STD?

The plain-English definition

An STD is a health condition you catch mainly through sexual contact. The germs involved live in semen, vaginal fluids, rectal fluids, blood, and on skin around the genitals, mouth and anus. When one of those germs moves between partners during sex — or through close intimate touch — it can establish an infection.

Doctors recognize more than 30 different sexually transmitted pathogens, according to the World Health Organization. Some are bacteria your antibiotics can eliminate completely (chlamydia, gonorrhea, syphilis). Some are viruses that stay in your body but are manageable (herpes, HIV) or preventable by vaccine (HPV, hepatitis B). And some are parasites or protozoa you can wash out entirely, like pubic lice ("crabs") and trichomoniasis.

The main types of STDs

STDs sort into four biological families:

• Bacterial: chlamydia, gonorrhea, syphilis — curable with antibiotics, though gonorrhea is developing resistance.
• Viral: herpes (HSV-1/HSV-2), HIV, human papillomavirus (HPV), hepatitis B and C, molluscum contagiosum — most can't be fully eliminated once established, but vaccines prevent several and antivirals control the rest.
• Parasitic/protozoal: trichomoniasis (a single-celled parasite), pubic lice and scabies (tiny arthropods) — all treatable.
• Fungal/bacterial overgrowths linked to sex: yeast infections and bacterial vaginosis aren't strictly STDs, but sexual activity influences them.

Our ranked overview covers the most common STDs with case counts.

How STDs actually spread

The classic route is unprotected vaginal or anal sex. But transmission doesn't require intercourse: oral sex passes gonorrhea, syphilis, herpes and HPV effectively; deep kissing transmits HSV-1; genital skin-to-skin contact spreads herpes, HPV and pubic lice even with a condom on. A pregnant person can also pass syphilis, HIV, hepatitis B or herpes to their baby during pregnancy or delivery.

What STDs don't do: jump from toilet seats, hugs, handshakes, shared plates or swimming pools. See the science behind the toilet-seat myth.

What an STD looks and feels like

Here's the uncomfortable truth: most STDs announce themselves quietly or not at all. Chlamydia is symptom-free in roughly 70% of women and half of men. Herpes spends most of its life dormant. Early HIV mimics the flu. When symptoms do appear they typically include:

• Unusual discharge from the penis, vagina or anus
• Burning or stinging when urinating
• Sores, blisters, warts or ulcers on the genitals, mouth or anus
• Rash — notably on palms and soles with secondary syphilis
• Pelvic pain in women; testicular ache in men
• Bleeding between periods or after sex

We break down every sign in detail in Signs of an STD and How Do You Know If You Have an STD?

Testing and treatment: the good news

Nearly everything on this page is detectable with simple tests — urine samples, quick swabs or a vial of blood. Results usually return within one to three days. Bacterial and parasitic STDs cure completely with short courses of medication; viral ones become chronic but respond well to suppressive therapy, and HIV treatment today drives the virus to undetectable levels.

If you're due for a check-up, compare every option — clinics, private labs, urgent care and home kits — in Where to Get Tested for STDs, then see how soon after exposure each test turns positive in our testing window chart.