More than 30 different bacteria, viruses, and parasites are known to spread through sexual contact. That figure, maintained by the World Health Organization, covers pathogens transmitted through vaginal, anal, and oral sex. But the real number depends on how broadly you define “sexually transmitted,” because several additional infections that primarily spread other ways can also pass between sexual partners.
What the 30-Plus Number Includes
The WHO’s count of more than 30 pathogens covers three categories: bacteria, viruses, and parasites. The most widely tracked include chlamydia, gonorrhea, syphilis, trichomoniasis, HIV, genital herpes, and human papillomavirus (HPV). These account for the vast majority of infections worldwide and are the ones routinely included in screening panels.
Beyond those headline names, the count includes less familiar infections like chancroid (a bacterial ulcer disease), lymphogranuloma venereum (a specific aggressive form of chlamydia), pubic lice, and scabies. Mycoplasma genitalium, a bacterium only recently added to clinical guidelines, is responsible for an estimated 15 to 20 percent of urethral infections in men that aren’t caused by gonorrhea. In women, it roughly doubles the risk of cervicitis, pelvic inflammatory disease, and preterm delivery. It’s a good example of how the list keeps evolving as diagnostic tools improve.
Infections That Blur the Line
Some pathogens don’t fit neatly into the “STI” category because sex is one transmission route among several. HIV and hepatitis B are the most prominent examples. Both spread through blood and other bodily fluids, but sexual contact is a major route for each. The CDC lists them alongside traditional STIs while noting they are “sometimes but not always spread by sexual transmission.”
Then there are infections most people associate with food or water contamination that also pass between sexual partners, particularly through oral-anal contact. These include Shigella, Salmonella, Campylobacter, Cryptosporidium, E. coli, and the parasite Giardia. Outbreaks of Shigella and Cryptosporidium have been documented specifically among men who have sex with men. If you count every pathogen capable of sexual transmission, the total climbs well beyond 30.
Even viruses like Zika and Ebola can spread sexually. Zika virus persists in semen far longer than in other bodily fluids. Potentially infectious virus has been detected in semen up to 69 days after symptoms begin, and viral genetic material lingers for an average of 54 days. About 7 percent of infected people still have detectable viral material in semen past 90 days. These aren’t classic STIs, but they are sexually transmissible.
Why the Number Keeps Growing
The list of recognized STIs has expanded over the past two decades, not because new pathogens are appearing out of nowhere, but because testing has gotten dramatically better. Mycoplasma genitalium is a perfect case study. The bacterium is so slow-growing that culturing it in a lab can take up to six months, which is why it went unrecognized for so long. Modern molecular testing can now identify it in hours.
Multiplex testing panels, which screen a single sample for multiple pathogens at once, have made it practical to look for infections that previously required separate, expensive tests. Some panels now detect seven or more pathogens from one swab. As these tools become standard, infections that were historically underdiagnosed are showing up in the data more frequently.
The Most Common vs. the Rare
Of the 30-plus known pathogens, a handful dominate global case counts. Chlamydia, gonorrhea, syphilis, and trichomoniasis are the four major curable STIs tracked by the WHO, collectively causing hundreds of millions of new infections each year. HPV and genital herpes are the most common viral STIs, with HPV infecting most sexually active people at some point in their lives.
At the other end of the spectrum, some STIs are genuinely rare in most parts of the world. Chancroid, once a significant cause of genital ulcers in tropical regions, has become uncommon in high-income countries. Lymphogranuloma venereum (LGV) remains relatively rare but is climbing. In 2023, 22 European countries reported 3,075 LGV cases, a 41 percent increase over the previous year. The Netherlands and Spain accounted for 77 percent of those cases, and nearly all occurred among men who have sex with men.
Drug Resistance Is Changing the Landscape
Knowing how many STIs exist matters less than understanding how treatable they remain. Mycoplasma genitalium is developing resistance to first-line antibiotics at an alarming pace. Molecular markers for resistance to the most commonly prescribed antibiotic for this infection are present in 44 to 90 percent of samples tested across the United States, Canada, Western Europe, and Australia. Even when the infection starts out susceptible to treatment, a standard dose of that antibiotic can push the bacteria toward resistance in 10 to 12 percent of cases.
Gonorrhea has followed a similar trajectory, progressively developing resistance to nearly every antibiotic class used against it. This pattern means the practical number of “easily curable” STIs is shrinking even as the total count of known pathogens grows. An infection that was once a simple course of pills can become a complicated, multi-step treatment challenge.
What the Number Actually Means for You
The 30-plus figure is useful as a reminder that STIs are far more varied than the handful most people hear about. Standard screening panels typically test for five to seven infections. That means a clean test result covers the most common threats but not every possible one. If you have symptoms that don’t match the usual suspects, a broader panel or specific testing for less common pathogens like Mycoplasma genitalium may be worth discussing.
The practical takeaway: the number of sexually transmissible pathogens is at least 30 by the WHO’s conservative count, likely closer to 40 or more if you include enteric bacteria and viruses with documented sexual transmission routes. The list is not fixed. It expands as diagnostics improve and as patterns of transmission shift across populations.