More than 30 different bacteria, viruses, and parasites are known to spread through sexual contact. That number surprises most people, who can usually name only a handful. Of those 30-plus pathogens, eight cause the vast majority of infections worldwide, and only four of them are currently curable.
The Eight Most Common STIs
The World Health Organization identifies eight pathogens responsible for the highest number of sexually transmitted infections globally. They fall into three categories based on what causes them.
Bacterial (curable):
- Chlamydia
- Gonorrhea
- Syphilis
Parasitic (curable):
- Trichomoniasis
Viral (not curable, but manageable):
- HIV
- Herpes simplex virus (HSV)
- Human papillomavirus (HPV)
- Hepatitis B
The four curable infections, chlamydia, gonorrhea, syphilis, and trichomoniasis, can generally be cleared with a single dose of antibiotics. The four viral infections stay in the body long-term. Antiviral medications can suppress symptoms and reduce transmission, but they don’t eliminate the virus entirely. Vaccines exist for HPV and hepatitis B, which makes prevention possible even though treatment isn’t a cure.
Beyond the Big Eight
The remaining 20-plus sexually transmitted pathogens include infections most people have never heard of. Some cause disease primarily in specific populations or under specific circumstances. A few examples: pubic lice, scabies, molluscum contagiosum, and hepatitis A and C can all spread through sexual contact, though some of these also transmit through non-sexual routes.
Newer infections are also gaining attention. Shigella, a bacterial infection that causes bloody diarrhea, fever, and abdominal pain, has been increasingly documented as sexually transmitted, particularly among men who have sex with men. Symptoms typically appear one to four days after exposure. Strains resistant to multiple antibiotics have emerged, sometimes limiting treatment options to intravenous antibiotics reserved for severe cases. Mycoplasma genitalium, another bacterium, has been recognized more recently as a sexually transmitted pathogen that can cause urethritis and pelvic inflammatory disease, and antibiotic resistance is a growing concern there as well.
Curable vs. Incurable: What That Means
The distinction between curable and incurable STIs matters more than the total count. If you contract chlamydia, gonorrhea, syphilis, or trichomoniasis, a course of antibiotics eliminates the infection. You can get reinfected, but each episode is treatable. The challenge is that antibiotic resistance is making some of these infections harder to treat, particularly gonorrhea, where resistant strains have been spreading globally.
The four viral STIs behave differently. HIV, once a death sentence, is now manageable with daily medication. An estimated 40.8 million people were living with HIV at the end of 2024, and about 1.3 million people acquired HIV that year. Deaths from HIV-related causes dropped to 630,000 in 2024, a significant decline from prior decades. Herpes and HPV are far more widespread but typically less dangerous, though HPV causes cervical and other cancers in a subset of infected people, and herpes can cause painful recurring outbreaks.
Most Infections Show No Symptoms
One reason STIs spread so effectively is that the majority of infected people don’t know they’re infected. Among women in studies across low- and middle-income countries, roughly 61% of chlamydia infections, 53% of gonorrhea infections, and 57% of trichomoniasis infections produced no symptoms at all. Herpes and HPV are similarly silent in many carriers. This means relying on symptoms to know whether you have an STI is unreliable. Testing is the only way to know for sure.
Asymptomatic infections aren’t harmless just because they’re painless. Untreated chlamydia and gonorrhea can lead to pelvic inflammatory disease, chronic pain, and infertility. Untreated syphilis progresses through stages that can eventually damage the brain, heart, and other organs. HPV can quietly trigger cellular changes that develop into cancer over years or decades.
STI Rates Vary by Region
Where you live influences your statistical risk. A large study comparing over 50,000 participants found that people in North America, Asia, and the Middle East had significantly higher reported STI rates compared to Europe. Latin America had lower rates than Europe. Living in a rural area was also associated with lower rates, likely reflecting fewer sexual partners on average and different patterns of social mixing.
These numbers come with caveats. In Africa, reported rates may appear lower partly because cultural stigma discourages disclosure. In the Middle East, some study populations overrepresent migrant workers living in conditions that elevate risk. Education also plays a role: people without a university degree had about 32% higher odds of reporting an STI, likely reflecting differences in access to health information and preventive services rather than behavior alone.
Why the Number Keeps Growing
The count of “more than 30” has gradually increased as diagnostic tools improve and researchers identify new transmission routes. Infections like Shigella were long considered foodborne or waterborne illnesses, but sexual transmission is now well-documented. Better PCR testing has made it possible to detect pathogens like Mycoplasma genitalium that older lab methods missed entirely. The total number of sexually transmitted pathogens will likely continue to rise, not because new diseases are appearing, but because science is getting better at recognizing how existing infections spread.