More than 30 different bacteria, viruses, parasites, and fungi are known to spread through sexual contact. That number, maintained by the World Health Organization, covers every pathogen that can transmit during vaginal, anal, or oral sex. But the picture is even more complex than a single number suggests, because one of those pathogens alone, HPV, is actually a group of more than 200 related viruses.
The Major Categories of STIs
The 30-plus sexually transmitted pathogens fall into four broad groups: bacteria, viruses, parasites, and fungi. The infections most people have heard of represent only a fraction of the total list, but they account for the vast majority of cases worldwide.
Bacterial STIs include chlamydia, gonorrhea, syphilis, and Mycoplasma genitalium (sometimes called Mgen). Bacterial vaginosis, while not always sexually transmitted, is closely linked to sexual activity and is tracked alongside traditional STIs. These infections are generally curable with antibiotics, though drug-resistant gonorrhea is a growing concern.
Viral STIs include HPV, genital herpes (caused by herpes simplex virus), HIV, and hepatitis B. Viral infections typically cannot be cured, but they can be managed or, in the case of HPV, often cleared by the immune system over time. Vaccines exist for both HPV and hepatitis B.
Parasitic STIs are led by trichomoniasis, caused by a single-celled organism. It is one of the most common STIs globally and is curable with medication. Pubic lice and scabies also spread through close body contact.
Why the Number Is Larger Than Most People Think
When people think of STIs, they usually picture a short list of five or six infections. The reason the actual count exceeds 30 is that many lesser-known pathogens also transmit sexually, even if sexual contact isn’t their primary route. Hepatitis A and certain strains of Shigella bacteria, for example, can spread during oral or anal sex. Some of these infections are better known in other contexts, like gastrointestinal illness, which is why they don’t always come to mind as STIs.
HPV alone complicates the count significantly. It is a group of more than 200 related viruses, of which 12 are classified as high-risk types linked to cancers of the cervix, throat, anus, and other areas. Low-risk HPV types can cause genital warts but rarely lead to cancer. So depending on how you count, “HPV” is either one STI or dozens.
How Common Are These Infections?
A handful of STIs dominate the global numbers. Trichomoniasis, chlamydia, gonorrhea, and syphilis are the four most common curable STIs. Among viral infections, HPV and herpes are extraordinarily widespread. Individuals can carry multiple infections at the same time, which makes the true burden even higher than case counts for any single pathogen suggest.
One reason these numbers stay so high is that most STIs produce no symptoms at all in many of the people who carry them. Chlamydia, for instance, causes no noticeable symptoms in about 50% of men and 70% of women. Gonorrhea is asymptomatic in up to 40% of men and at least 50% of women. HPV is the most silent of all: 70 to 90% of people with the virus never develop any symptoms and may never know they have it. This means infections spread easily between people who feel perfectly healthy.
How They Spread Beyond Penetrative Sex
Not all STIs require intercourse to transmit. Herpes, syphilis, and HPV can spread through brief skin-to-skin contact when your genitals or mouth touch an infected person’s genitals, mouth, or anal area. Syphilis and herpes are especially likely to transmit when open sores are present, but herpes can also shed from skin that looks completely normal.
Oral sex carries its own set of risks. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all transmit through oral-genital contact. Hepatitis B may also spread during oral sex if menstrual blood is involved and the other person has cuts or sores in the mouth. Even kissing can transmit oral herpes (HSV-1), which can then move to the genital area through oral sex.
Touching a sore and then touching your own genitals, mouth, or eyes before washing your hands is another, less obvious route. These non-penetrative pathways are a major reason that barrier methods like condoms reduce but do not eliminate STI risk for infections that live on surrounding skin.
What Happens When STIs Go Untreated
Because so many infections are silent, they often go untreated for months or years. The consequences depend on the specific pathogen. Untreated chlamydia and gonorrhea can lead to pelvic inflammatory disease in women, which in turn can cause chronic pelvic pain, ectopic pregnancy, and infertility. Untreated syphilis progresses through stages over years and can eventually damage the brain, heart, and other organs.
HPV’s high-risk strains are responsible for nearly all cervical cancers and a significant share of throat and anal cancers. These cancers typically develop over a decade or more, which is why routine screening catches precancerous changes early enough to treat them. Herpes, while not life-threatening for most adults, can cause serious complications in newborns if transmitted during delivery.
HIV remains the most consequential viral STI in terms of long-term health impact, though modern antiretroviral therapy allows most people with HIV to live a normal lifespan and reach an undetectable viral load that prevents transmission to partners.
Which STIs Are Curable and Which Are Not
Bacterial and parasitic STIs are generally curable. Chlamydia, gonorrhea, syphilis, Mgen, and trichomoniasis all respond to the right antibiotics or antiparasitic medications, though treatment works best when infections are caught early. Drug resistance is an increasing problem, particularly with gonorrhea, which has developed resistance to multiple classes of antibiotics over the past few decades.
Viral STIs follow a different pattern. There is no cure for herpes, HIV, or hepatitis B, though all three can be managed with medication that controls symptoms and reduces transmission. HPV has no direct treatment, but the immune system clears most HPV infections within one to two years. The HPV vaccine, which targets the highest-risk strains, has dramatically reduced infection rates and HPV-related cancers in countries with high vaccination coverage. Hepatitis B is also preventable through vaccination, making it one of the few viral STIs you can protect against before any exposure occurs.