More than 30 different bacteria, viruses, and parasites are known to spread through sexual contact, but eight pathogens account for the vast majority of infections worldwide. The World Health Organization groups these eight into two categories: four that are curable with antibiotics and four viral infections that can be managed but not eliminated from the body.
The Eight Most Common STIs
The four curable STIs are chlamydia, gonorrhea, syphilis, and trichomoniasis. Together, these four caused an estimated 374 million new infections globally in 2020. Trichomoniasis led the count at 156 million new cases, followed by chlamydia at 129 million, gonorrhea at 82 million, and syphilis at 7.1 million. All four can be treated and fully cleared with antibiotics, though gonorrhea has become increasingly resistant to standard treatments in recent years.
The four viral STIs are herpes simplex virus (HSV), human papillomavirus (HPV), hepatitis B, and HIV. These infections can be controlled with medication but stay in the body long-term. The numbers are staggering: more than 520 million people were living with genital herpes in 2020, roughly 300 million women had an HPV infection, and 254 million people were living with chronic hepatitis B in 2022.
Beyond the Big Eight
The eight major STIs get the most attention, but they’re far from the complete list. Mycoplasma genitalium, sometimes called Mgen, is a bacterial infection increasingly recognized as a significant STI. It can infect the cervix, urethra, or rectum and is curable with antibiotics, though drug-resistant strains are becoming more common. Mgen often causes no symptoms, which means many people carry and spread it without knowing.
Hepatitis C, while primarily spread through blood, can also be transmitted sexually, particularly among men who have sex with men. Pubic lice (crabs) and scabies spread through close skin-to-skin contact during sex. Molluscum contagiosum, a viral skin infection, also passes through sexual contact in adults. And certain gut infections, including those caused by Shigella and Campylobacter bacteria, can spread during sexual activity that involves direct or indirect fecal-oral contact. Drug-resistant strains of Shigella have been linked to outbreaks in several communities.
Why the Number Keeps Growing
The total count of sexually transmitted pathogens isn’t fixed. As diagnostic testing improves, infections that were previously hard to detect get added to the list. Mgen is a good example: the bacterium was identified decades ago, but reliable testing only became widely available in recent years. Some infections that were traditionally classified under other categories, like certain gut pathogens, are now recognized as having significant sexual transmission routes.
How Many People Are Affected
In the United States alone, provisional 2024 data from the CDC counted more than 2.2 million reported cases of just three STIs: about 1.5 million chlamydia cases, 543,000 gonorrhea cases, and 190,000 syphilis cases. These numbers only capture infections that were tested for, diagnosed, and reported. The true burden is much higher because many STIs produce no symptoms at all.
Chlamydia and gonorrhea in particular often cause no noticeable symptoms, especially in women. HPV clears on its own in most people without ever being detected. Herpes can cause mild or infrequent outbreaks that people may not recognize as an STI. This silent nature is a major reason STIs spread so effectively, and why routine screening catches infections that waiting for symptoms never would.
Which STIs Are Curable and Which Aren’t
Bacterial and parasitic STIs are generally curable. Chlamydia, gonorrhea, syphilis, trichomoniasis, and Mgen all respond to antibiotics, though the right antibiotic matters. Gonorrhea and Mgen in particular have developed resistance to several commonly used drugs, which can make treatment more complicated.
Viral STIs follow a different pattern. Herpes can be suppressed with daily antiviral medication, reducing both outbreaks and the risk of spreading the virus. HIV can be controlled so effectively with antivirals that the virus becomes undetectable in the blood, dramatically lowering transmission risk. But stopping medication allows both viruses to become active again. HPV has no direct treatment, though the immune system clears most infections within a couple of years, and vaccines prevent the strains most likely to cause cancer. Hepatitis B can also be managed with antivirals and prevented with vaccination.
What Happens When STIs Go Untreated
The reason the number of STIs matters is that each one carries real health consequences if left undiagnosed. Untreated chlamydia or gonorrhea can lead to pelvic inflammatory disease (PID) in women, a condition that causes scarring in the fallopian tubes. One in eight women with a history of PID has difficulty getting pregnant. PID can also cause chronic pelvic pain and increase the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus.
Untreated syphilis progresses through stages over years and can eventually damage the brain, heart, and other organs. Certain strains of HPV cause cervical, throat, and anal cancers. Chronic hepatitis B can lead to liver failure. And untreated HIV progressively destroys the immune system. Many of these outcomes are preventable with early detection and treatment, which is why knowing how many STIs exist, and that most of them can be silent, changes how you think about screening.
Testing Windows to Know
If you’ve had a potential exposure, the timing of your test matters. Each infection has a “window period” before it becomes detectable. HIV shows up on a blood test (antigen/antibody) within about two weeks for most people, with nearly all infections detectable by six weeks. An oral swab for HIV takes longer: one month catches most cases, three months catches almost all. Syphilis follows a similar timeline, with blood tests reliable after about one month and definitive by three months.
Hepatitis B typically becomes detectable within three to six weeks. Hepatitis C takes longer, with two months catching most infections but six months needed to rule it out completely. Chlamydia and gonorrhea can usually be detected within one to two weeks of exposure. Testing too early can produce a false negative, so if your first test comes back clean but the exposure was recent, a follow-up test after the full window period gives you a more reliable answer.