STDs (sexually transmitted diseases) are caused by bacteria, viruses, or parasites that pass from one person to another, primarily through sexual contact. These pathogens enter the body through the thin, moist tissue lining the genitals, mouth, and rectum, exploiting microscopic tears that occur naturally during sex. More than 2.2 million cases of just three STDs (chlamydia, gonorrhea, and syphilis) were reported in the United States in 2024 alone, and the actual number of infections is far higher since many never get diagnosed.
How STD Pathogens Enter the Body
The surfaces inside your genitals, anus, and mouth are lined with mucous membranes, tissue that’s much thinner and more vulnerable than regular skin. During vaginal, anal, or oral sex, tiny abrasions form in this tissue. STD-causing organisms slip through those micro-tears and into the underlying cells or bloodstream. Different pathogens use different strategies once inside: the bacteria behind gonorrhea, for instance, uses glucose to penetrate the cells lining the genitals, while HPV targets a specific layer of skin cells and begins replicating there. The parasite that causes trichomoniasis takes a more aggressive approach, directly damaging the tissue lining the vagina, cervix, and urethra to create tiny ulcerations that help it establish infection.
Once inside, many of these organisms have evolved ways to hide from your immune system. HIV, HPV, and herpes all interfere with the way infected cells signal for help, effectively making themselves invisible to your body’s defenses. This is one reason viral STDs can persist for years or even a lifetime.
Bacterial STDs
Bacteria cause several of the most common STDs. Chlamydia is the single most reported bacterial STD in the U.S., with over 1.5 million cases in 2024. It infects the cervix in women and the urethra in both men and women, and can also infect the rectum. Gonorrhea, the second most common, accounted for about 543,000 cases in 2024. Both infections can travel deeper into the reproductive tract if untreated, potentially causing pelvic inflammatory disease and infertility.
Syphilis, caused by a corkscrew-shaped bacterium, progresses through distinct stages over months or years if left untreated, eventually affecting the heart, brain, and other organs. While primary and secondary syphilis cases actually dropped 22% from 2023 to 2024, congenital syphilis (passed from mother to baby during pregnancy) rose for the twelfth consecutive year, reaching nearly 4,000 cases. A lesser-known bacterium called Mycoplasma genitalium is now recognized as a significant cause of persistent urethral infections in men and is linked to cervicitis, pelvic inflammatory disease, and infertility in women.
Viral STDs
HPV is the most common sexually transmitted infection in the United States overall, and most people who have it never know. The virus has dozens of strains: some cause genital warts, others can lead to cervical, throat, and anal cancers over time. Herpes simplex virus comes in two types. HSV-1 traditionally causes oral cold sores but increasingly causes genital infections through oral sex, while HSV-2 is the classic cause of genital herpes. Neither type can be cured, though outbreaks become less frequent over time for most people.
HIV destroys immune cells, and without treatment it progresses to AIDS. Hepatitis B and C both cause liver inflammation and can be transmitted sexually, though hepatitis C spreads more efficiently through blood. Mpox, which spreads through skin-to-skin contact including sexual contact, has gained attention more recently as a sexually transmitted pathogen.
Parasites and Other Organisms
Trichomoniasis is the most common parasitic STD, caused by a single-celled organism that infects the vagina, cervix, and urethra. It’s highly treatable with antibiotics but frequently goes undiagnosed because symptoms can be mild or absent entirely.
Two ectoparasites, organisms that live on the body’s surface, also spread through sexual contact. Pubic lice (sometimes called “crabs”) attach to coarse body hair and cause intense itching. Scabies mites burrow into the skin, creating a characteristic rash with visible tracks, particularly in the finger webs and genital area. Oral-anal sexual contact can also transmit intestinal parasites like Giardia and Entamoeba, which cause gastrointestinal symptoms rather than genital ones.
Different Types of Sexual Contact, Different Risks
Vaginal and anal sex carry the highest transmission risk for most STDs. Anal sex is particularly risky because the rectal lining is thinner than vaginal tissue and tears more easily, giving pathogens a direct route to the bloodstream. This is one reason HIV transmission rates are higher during anal sex than vaginal sex.
Oral sex is lower risk but far from risk-free. Chlamydia, gonorrhea, syphilis, herpes, and HPV can all spread through oral sex. In one study of men diagnosed with syphilis, 1 in 5 reported oral sex as their only sexual contact. HIV transmission through oral sex is possible but considered extremely low compared to vaginal or anal sex. Factors like bleeding gums, mouth sores, or open sores on the genitals may increase the risk during oral contact, though the exact degree isn’t well quantified. Oral-anal contact can transmit hepatitis A and B, as well as bacterial infections like Shigella and E. coli.
Non-Sexual Transmission Routes
Several STD pathogens can also spread without sexual contact. HIV, hepatitis B, and hepatitis C can pass through direct blood-to-blood contact, most commonly by sharing needles for drug injection, tattooing, or piercing with unsterilized equipment. Syphilis, HIV, and hepatitis B can all pass from a pregnant person to their baby during pregnancy, childbirth, or breastfeeding. Healthcare workers face a small risk from needlestick injuries involving contaminated blood. In all of these cases, the same pathogen is responsible whether transmission is sexual or not.
Why So Many Cases Go Undetected
One of the biggest reasons STDs spread so effectively is that the majority of infections produce no obvious symptoms. Research pooling data from multiple studies found that roughly 61% of chlamydia infections, 53% of gonorrhea infections, and 57% of trichomoniasis infections in women were completely asymptomatic. HPV and herpes are similarly silent in many people. This means someone can carry and transmit an infection for weeks, months, or even years without ever feeling sick. Asymptomatic carriers form a hidden reservoir that continuously fuels new infections, which is why routine screening matters even when you feel perfectly fine.
Factors That Increase Susceptibility
Not everyone faces the same risk of catching an STD from a given exposure. Age and anatomy play a role: young women and female adolescents are biologically more susceptible than men because the cervix in younger women has a larger area of delicate tissue that’s more easily penetrated by pathogens. This tissue gradually changes with age, offering somewhat more protection.
Having one STD already increases your risk of acquiring another. STDs that cause sores or inflammation, like herpes or syphilis, create openings in the skin and recruit immune cells to the area, both of which make it easier for other pathogens, including HIV, to gain entry. Multiple concurrent infections compound the problem, making treatment more complex and transmission more likely. Vaginal douching can disrupt the natural balance of protective bacteria, increasing vulnerability to infections like bacterial vaginosis and potentially making it easier for other STD pathogens to take hold.