What Are Sexually Transmitted Infections (STIs)?

Sexually transmitted infections are infections passed from one person to another primarily through sexual contact, including vaginal, anal, and oral sex. They are caused by bacteria, viruses, or parasites, and they range from curable nuisances to lifelong conditions that can lead to infertility, cancer, or death if left unmanaged. What makes STIs particularly tricky is that most of them produce no obvious symptoms for weeks, months, or sometimes ever, which means people spread them without knowing they are infected. Understanding the different types, how they behave in the body, and what modern medicine can and cannot do about them matters more now than it did a decade ago, partly because some STIs are becoming harder to treat.

Bacteria, Viruses, and Parasites

STIs fall into three broad categories based on what causes them, and the distinction matters because it determines whether the infection can be cured outright or only managed.

Bacterial STIs include chlamydia, gonorrhea, and syphilis. These are caused by living bacteria that can be killed with antibiotics, at least in principle. Chlamydia and gonorrhea can infect the throat, rectum, eyes, and urogenital tract in both men and women.¹ Syphilis, caused by the spiral-shaped bacterium Treponema pallidum, progresses through distinct stages if untreated. It starts with a painless ulcer, moves to a body-wide rash, and can eventually go dormant for years before causing serious organ damage.¹ Less commonly discussed bacterial STIs such as chancroid, granuloma inguinale, and certain Campylobacter and Shigella infections can also be transmitted sexually, especially through anal intercourse.²

Viral STIs include herpes simplex virus (HSV), human papillomavirus (HPV), HIV, and hepatitis B. Viruses cannot be killed the way bacteria can. Antiviral drugs suppress them, slow their replication, or prevent complications, but the virus typically remains in the body. HSV, for instance, establishes a lifelong latent infection in sensory neurons after the initial outbreak on skin or mucous membranes.³ HPV is recognized as the cause of cervical cancer and is increasingly linked with other cancers.⁴

Parasitic STIs are the third category. Trichomoniasis, caused by the single-celled parasite Trichomonas vaginalis, is the most common nonviral STI, affecting an estimated 3.7 million people in the United States alone.⁵ It is curable with a course of antibiotics, yet it remains remarkably under-studied compared to other STIs. Pubic lice and scabies are also considered sexually transmitted parasites, though they spread through close skin contact rather than exchange of bodily fluids.

Why Most STIs Go Unnoticed

The single most important thing to understand about STIs is that the majority of infections produce no symptoms. This is not a rare exception; it is the norm. More than 70% of urogenital chlamydia infections in women and more than 80% in men are asymptomatic. For gonorrhea, somewhere between 86% and 93% of urogenital infections in women are silent, and among both sexes, between 53% and 100% of infections at sites outside the genitals produce no symptoms at all.¹

Herpes follows a similar pattern. HSV-2, the type most associated with genital herpes, is subclinical in most people who carry it. The virus sheds from genital surfaces even without visible sores or any sensation that something is wrong.⁶ In one study, over half of women with genital HSV-2 had virus detected on days when they had no symptoms at all, with subclinical shedding occurring on about 2% of days monitored.⁷ People who know they have herpes shed more frequently than those who have never been diagnosed, but asymptomatic carriers still shed the virus on nearly 9% of days studied.⁸

The practical consequence is straightforward: you cannot tell whether you or a partner has an STI based on how either of you feels. Testing is the only reliable way to know.

Long-Term Damage to Reproductive Health

Left untreated, bacterial STIs can quietly damage the reproductive system. Most cases of tubal factor infertility, where scarring blocks or damages the fallopian tubes, are traced back to untreated chlamydia or gonorrhea that ascended from the cervix.⁹ In a large retrospective study, women who tested positive for chlamydia had roughly twice the risk of developing pelvic inflammatory disease compared to women who tested negative, along with a meaningfully higher risk of ectopic pregnancy and infertility.¹⁰ Women who tested positive more than once faced even higher risk, which underscores why reinfection is not just an inconvenience but a compounding threat.

Trichomoniasis, often dismissed as minor, also carries serious downstream consequences including associations with adverse pregnancy outcomes, infertility, and increased susceptibility to HIV.¹¹ HSV-2 shedding, even when subclinical, may cause genital tract inflammation that also raises HIV risk.⁶ These interconnections mean that one untreated STI can open the door to another, more dangerous one.

STIs and Cancer

HPV deserves special attention because it does not just cause genital warts. Certain high-risk strains are now established as the cause of cervical cancer and are linked to cancers of the throat, anus, penis, vagina, and vulva. The virus manipulates the cell cycle to replicate its own genome, and in doing so, it drives the kind of genomic instability that pushes cells toward becoming cancerous.⁴ HPV vaccination has proven remarkably effective at preventing these cancers. A nationwide Swedish cohort study found that among women born in years eligible for a school-based HPV vaccination program, the rate of high-grade cervical lesions dropped by about half compared to older cohorts, and this benefit extended even to unvaccinated women in the same age group due to herd effects.¹²

Passing STIs to Babies

STIs are not limited to sexual transmission. Syphilis, in particular, can cross the placenta and infect a developing fetus at any point during pregnancy, with the highest risk during the primary and secondary stages of the mother’s infection.¹³ Congenital syphilis can cause stillbirth, bone deformities, and neurological damage. Cases have been surging in several countries. In Portugal, for example, the incidence of congenital syphilis rose eightfold between 2016 and 2024, even among mothers whose pregnancies were formally monitored, highlighting breakdowns in the chain from screening to treatment.¹⁴

Gonorrhea and chlamydia can also be transmitted to infants during vaginal delivery. Gonococcal conjunctivitis in newborns is a known risk, and chlamydia acquired at birth can cause eye infections or pneumonia.¹ Routine prenatal screening exists to catch these infections, but as the Portuguese data illustrate, screening alone does not guarantee treatment follows.

Where You Test Matters

Standard STI screening has historically focused on urine samples or genital swabs, but infections at the throat and rectum are easy to miss with that approach. Research has found that over 80% of rectal and pharyngeal chlamydia cases and over 65% of rectal and pharyngeal gonorrhea cases would go undetected if only genital testing was performed.¹⁵ Men with infections at these extragenital sites were no more likely to have symptoms than men with no infection at all, which means relying on symptoms to guide testing is a dead end.¹⁶

Nucleic acid amplification tests are the preferred method for detecting chlamydia and gonorrhea, though they have not been formally cleared by the FDA for use on rectal or throat swabs despite being widely used for those sites in practice.¹⁷ Point-of-care tests that deliver results in about 30 minutes are also emerging, with one chlamydia test demonstrating over 96% sensitivity and nearly 98% specificity in a multi-center trial.¹⁸ Faster testing matters because people are more likely to get treated promptly if they receive results on the spot rather than waiting days for a lab report.

Prevention Beyond Condoms

Condoms remain the most widely available barrier method and offer strong protection. They reduce the risk of HIV, hepatitis B, and gonorrhea by more than 90% when used consistently.¹⁹ They are less effective against infections that spread through skin-to-skin contact in areas the condom does not cover, such as herpes and HPV, though they still reduce the probability.

HPV vaccination is the standout success story. When vaccination coverage is high enough, the benefits ripple through the population. The Swedish data showed that even unvaccinated women in vaccinated age cohorts saw substantially fewer high-grade cervical lesions, a clear sign that the virus circulates less when enough people are immune.¹²

A newer strategy is doxycycline post-exposure prophylaxis, or doxy-PEP: taking a single dose of the antibiotic doxycycline within 72 hours after unprotected sex. In a randomized trial among men who have sex with men and transgender women, those who took doxy-PEP had about a third the STI rate compared to those receiving standard care. Chlamydia and syphilis dropped the most steeply, while gonorrhea showed a smaller reduction.²⁰ A separate randomized trial confirmed similar results, finding about a 60% reduction in overall STI diagnoses among doxy-PEP users.²¹ Real-world implementation data from San Francisco showed chlamydia and early syphilis cases among the target population falling by roughly half within about a year of rollout, though gonorrhea cases actually ticked upward.²² That gonorrhea exception is a red flag, and it connects to a broader concern.

The Antibiotic Resistance Problem

Gonorrhea has steadily evolved resistance to one antibiotic class after another. It has burned through sulfonamides, penicillins, tetracyclines, and fluoroquinolones over the decades. The last reliable first-line drug is ceftriaxone, an injectable cephalosporin, and strains with high-level resistance to even that drug have already been identified.²³ Researchers have described the gonococcus as evolving into a genuine “superbug” that may render some gonorrhea infections functionally untreatable.²⁴

This backdrop gives weight to the concern about widespread doxy-PEP use. Doxycycline is a tetracycline-class antibiotic, and mass use of any antibiotic in a population generates selection pressure for resistant organisms. The real-world data already show that gonorrhea does not respond to doxy-PEP the way chlamydia and syphilis do, which is consistent with the fact that many gonorrhea strains are already resistant to tetracyclines. The worry is not only that doxy-PEP will fail against gonorrhea but that it could accelerate resistance in other organisms as well. Clinicians prescribing doxy-PEP are weighing the clear short-term benefit against an uncertain but plausible long-term cost.

The Vaginal Microbiome and Susceptibility

Not everyone exposed to an STI gets infected, and the composition of the vaginal microbiome appears to play a role. Bacterial vaginosis, a condition where the normal balance of vaginal bacteria is disrupted, is associated with higher rates of STI acquisition. In one study, women with bacterial vaginosis had significantly more STI co-infections involving two different pathogens compared to women without it.²⁵ Bacterial vaginosis itself is not classified as an STI, but it seems to create conditions where STI-causing organisms find it easier to establish themselves. Whether that relationship is purely about the local environment becoming more hospitable to invaders, or whether the same sexual behaviors drive both conditions, remains an active area of investigation.

Treating the Partner, Not Just the Patient

One of the most frustrating patterns in STI management is reinfection. You get treated, your untreated partner re-exposes you, and the cycle starts over. Expedited partner therapy is a strategy where the diagnosed patient is given medication or a prescription to pass along to their sexual partner without requiring the partner to come in for a separate clinic visit. This approach has been shown to produce reinfection rates that are equivalent to or better than the traditional approach for chlamydia, gonorrhea, and trichomoniasis.²⁶ In practice, however, the impact is modest. A study among female adolescents found that reinfection rates were lower among patients who accepted expedited partner therapy compared to those who declined, but the differences did not reach statistical significance, in part because uptake was limited.²⁷ The tool works in theory; getting people to actually use it is the bottleneck.

Why Vaccines for Bacterial STIs Are So Difficult

Given how well HPV vaccination has worked, a natural question is why there are no vaccines for chlamydia, gonorrhea, or syphilis. The short answer is that these bacteria have evolved sophisticated ways to dodge the immune system. Chlamydia, gonorrhea, and syphilis all present major challenges for vaccine development because the immune responses they provoke in the female genital tract tend to be partial, temporary, or actively subverted by the pathogen.²⁸ You can get chlamydia, clear it, and get it again, which tells you that natural infection does not produce reliable lasting immunity. Vaccine researchers essentially have to do better than the body’s own immune response, which is a fundamentally harder problem than, say, building a vaccine against a virus that the body naturally develops good antibodies to. Candidate vaccines are in various stages of research, but none is close to clinical use.

Syphilis adds its own wrinkle. Treponema pallidum is extraordinarily difficult to grow in a lab, which has slowed basic research into its biology for over a century. And gonorrhea’s rapid acquisition of antibiotic resistance hints at a pathogen that evolves quickly in response to selective pressure, a trait that would likely complicate any vaccine strategy that targets surface proteins the bacterium can mutate away from.

1
JAMA. Diagnosis and Treatment of Sexually Transmitted Infections: A Review
2
PubMed Central. Bacterial sexually transmitted diseases
3
PubMed Central. Mechanisms of herpes simplex virus latency and reactivation
4
PubMed Central. Molecular mechanisms of human papillomavirus-induced carcinogenesis
5
PubMed Central. Trichomoniasis: the “neglected” sexually transmitted disease
6
PubMed Central. Current Concepts for Genital Herpes Simplex Virus Infection: Diagnostics and Pathogenesis of Genital Tract Shedding
7
PubMed. Virologic characteristics of subclinical and symptomatic genital herpes infections
8
PubMed Central. Genital Shedding of Herpes Simplex Virus Among Symptomatic and Asymptomatic Persons with HSV-2 Infection
9
PubMed Central. Sexually transmitted diseases and infertility
10
Clinical Infectious Diseases. Chlamydia trachomatis and the Risk of Pelvic Inflammatory Disease, Ectopic Pregnancy, and Female Infertility: A Retrospective Cohort Study Among Primary Care Patients
11
PubMed Central. Trichomoniasis – are we giving the deserved attention to the most common non-viral sexually transmitted disease worldwide?
12
The Lancet. Long-term herd effects of HPV vaccination on high-grade cervical lesions: a nationwide register-based cohort study in Sweden
13
PubMed Central. Syphilis in pregnancy: A practical guide for prenatal care providers
14
PubMed Central. Congenital syphilis in Portugal, 2015-2024: Temporal-spatial characterization and gaps in prevention of vertical transmission
15
PubMed Central. High proportions of rectal and pharyngeal chlamydia and gonorrhea cases among cisgender men are missed using current CDC screening recommendations
16
PubMed Central. Patterns of Extragenital Chlamydia and Gonorrhea in Women and Men Who Have Sex with Men Reporting a History of Receptive Anal Intercourse
17
PubMed Central. Use of Nucleic Acid Amplification Testing for Diagnosis of Extragenital Sexually Transmitted Infections
18
PubMed Central. A 30-Min Nucleic Acid Amplification Point-of-Care Test for Genital Chlamydia trachomatis Infection in Women: A Prospective, Multi-center Study of Diagnostic Accuracy
19
PubMed Central. Condoms: Past, present, and future
20
PubMed Central. Postexposure Doxycycline to Prevent Bacterial Sexually Transmitted Infections
21
The Lancet Infectious Diseases. Doxycycline post-exposure prophylaxis for the prevention of bacterial sexually transmitted infections: a randomized open-label trial
22
PubMed Central. Doxycycline Postexposure Prophylaxis and Sexually Transmitted Infection Trends
23
PubMed Central. Antibiotic resistance in Neisseria gonorrhoeae: origin, evolution, and lessons learned for the future
24
PubMed Central. Emergence of multidrug-resistant, extensively drug-resistant and untreatable gonorrhea
25
PubMed Central. Relationship between Bacterial Vaginosis and Sexually Transmitted Infections: Coincidence, Consequence or Co-Transmission?
26
PubMed. A Review of Expedited Partner Therapy for the Management of Sexually Transmitted Infections in Adolescents
27
Sexually Transmitted Diseases. Expedited Partner Therapy in Female Adolescents: A Study of Acceptance and the Impact on Reinfection Rates
28
PubMed Central. Immunity to Sexually Transmitted Bacterial Infections of the Female Genital Tract: Toward Effective Vaccines