Sexually transmitted infections fall into three broad categories based on the pathogen involved: bacterial (chlamydia, gonorrhea, syphilis), viral (HIV, HPV, herpes, hepatitis B), and parasitic (trichomoniasis, pubic lice, scabies). Each type behaves differently in the body, shows up on different tests, and responds to different treatments. What makes STIs collectively tricky is that many cause no symptoms at all, which means a person can carry and spread an infection without ever feeling sick.
Bacterial STIs
Bacterial infections are the most commonly discussed STIs because they are, in principle, curable with antibiotics. The major ones are chlamydia, gonorrhea, and syphilis. Chlamydia, caused by the bacterium Chlamydia trachomatis, is the most common bacterial STI worldwide and disproportionately affects younger adults.1PubMed. Bacterial sexually transmitted infections Gonorrhea, caused by Neisseria gonorrhoeae, often infects the same sites and can appear alongside chlamydia. Syphilis, caused by the spirochete Treponema pallidum, progresses through distinct stages over months to years and can affect the brain and cardiovascular system if left untreated.
Less familiar bacterial STIs include chancroid (caused by Haemophilus ducreyi), which produces painful genital ulcers, and granuloma inguinale, which causes progressive ulcerative lesions.2ScienceDirect. Molecular Medical Microbiology – Section: Abstract These are rare in high-income countries but remain significant in parts of Africa, Southeast Asia, and the Caribbean. Certain species of Mycoplasma and Ureaplasma also occasionally cause urethritis and cervicitis, though they receive less public attention.
Viral STIs
Viral STIs tend to be chronic rather than curable. The major ones include human papillomavirus (HPV), herpes simplex virus (HSV types 1 and 2), HIV, and hepatitis B. HPV is the most widespread: most sexually active people are exposed to it at some point. While most HPV infections clear on their own within a year or two, persistent infection with high-risk strains can promote cancer development by disabling the body’s tumor-suppressor mechanisms.3PubMed Central. Immune Dysregulation and Viral Persistence in HPV-HIV Coinfection: Oncogenic and Clinical Implications – Section: RESULTS Cervical cancer is the most well-known consequence, but HPV also drives cancers of the throat, anus, and penis.
Genital herpes, caused by HSV-2 (and increasingly HSV-1), produces recurrent outbreaks of painful blisters. It cannot be eliminated from the body, though antiviral medication reduces the frequency and severity of outbreaks and lowers the chance of transmission. HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiretroviral therapy has transformed HIV from a death sentence into a manageable chronic condition for most people with access to medication. Hepatitis B attacks the liver and, while a vaccine exists, chronic infection can lead to cirrhosis and liver cancer over decades.
Parasitic STIs
Parasitic STIs get less attention, but trichomoniasis is actually one of the most common STIs globally. It is caused by the single-celled protozoan Trichomonas vaginalis and typically produces vaginitis, with symptoms like itching, burning, and frothy discharge, though many infected people have no symptoms at all.4Primary Care: Clinics in Office Practice. Sexually Transmitted Parasitic Diseases – Section: SUMMARY Other protozoan infections that can be transmitted sexually include those caused by Entamoeba histolytica and Giardia duodenalis, particularly through oral-anal contact.5PubMed Central. Trichomoniasis and Other Sexually Transmitted Parasitic Diseases in Women – Section: Abstract
Ectoparasites round out the category. Pubic lice (Pthirus pubis) and scabies (Sarcoptes scabiei) both spread through close skin-to-skin contact and cause intense itching. Scabies typically shows up as a rash with linear burrows in the finger webs and genital area, while pubic lice are diagnosed by spotting the adult lice or their eggs on coarse body hair.4Primary Care: Clinics in Office Practice. Sexually Transmitted Parasitic Diseases – Section: SUMMARY Both are treatable with topical medications.
Why Most STIs Go Unnoticed
One of the most underappreciated facts about STIs is how often they cause no symptoms whatsoever. One estimate suggests that roughly three-quarters of chlamydia infections and close to half of gonorrhea infections never produce recognizable symptoms, and the vast majority of untreated cases remained untreated specifically because the person never felt sick.6PubMed. Asymptomatic sexually transmitted diseases: the case for screening – Section: RESULTS A systematic review looking at women in lower-income countries found that roughly 60% of chlamydia infections, about half of gonorrhea infections, and close to 60% of trichomoniasis infections were asymptomatic.7PLOS Global Public Health. Asymptomatic infections with Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis among women in low- and middle-income countries: A systematic review and meta-analysis – Section: Results
This has a straightforward practical implication: you cannot wait for symptoms to tell you whether you have an STI. Regular screening is the only reliable way to catch infections early, especially for chlamydia, gonorrhea, and HIV. Most public health guidelines recommend annual chlamydia screening for sexually active women under 25, and routine HIV testing at least once for all adults.
How STI Testing Works
Different infections require different testing methods. Chlamydia and gonorrhea are most commonly detected using nucleic acid amplification tests (NAATs), which look for the pathogen’s genetic material. These tests perform well on vaginal swabs, cervical swabs, and urine samples. A systematic review found that vaginal specimens achieved sensitivity above 80% for both infections in nearly all studies evaluated.8BMJ Open. Evaluation of the performance of nucleic acid amplification tests (NAATs) in detection of chlamydia and gonorrhoea infection in vaginal specimens relative to patient infection status: a systematic review – Section: Abstract For men who have sex with men, testing from multiple sites (throat, rectum, and urine) is important because infections at those locations often produce no symptoms. Pooling specimens from all three sites into a single test slightly reduces sensitivity compared with testing each site individually.9Sexually Transmitted Diseases. Screening for Chlamydia trachomatis and Neisseria gonorrhoeae Infections in Men Who Have Sex With Men – Section: Abstract
HIV testing has evolved considerably. Fourth-generation tests that detect both antibodies and viral antigen can identify infections much earlier than older antibody-only tests. Research on the detection window has shown that RNA-based testing can detect HIV about a month before older antibody tests, while antigen-based testing falls in between.10PubMed. Time course of detection of viral and serologic markers preceding human immunodeficiency virus type 1 seroconversion: implications for screening of blood and tissue donors – Section: RESULTS Using fourth-generation testing in routine screening protocols helps catch infections during the window period that earlier tests would miss.11PLoS ONE. Screening and diagnostic testing protocols for HIV and Syphilis infections in health care setting in Qatar: Evaluation and recommendations – Section: Results Syphilis is usually detected through blood tests that look for antibodies, and herpes through swab tests during an outbreak or type-specific blood tests.
Home Sampling and Self-Collection
Getting tested has historically meant going to a clinic, which isn’t always practical or comfortable. Home self-collection kits, where you take your own swab or urine sample and mail it to a lab, are gaining traction as an alternative. A large systematic review found that people randomized to self-collect at home were more than two and a half times as likely to complete testing for chlamydia and gonorrhea compared with those told to go to a clinic.12Sexually Transmitted Infections. At-home specimen self-collection as an additional testing strategy for chlamydia and gonorrhoea: a systematic literature review and meta-analysis – Section: Uptake of STI testing at home and in clinical settings A pilot program for HIV-positive men who have sex with men found that offering home sampling for chlamydia, gonorrhea, hepatitis B, and syphilis increased overall STI test uptake and was well accepted by both patients and providers.13PubMed Central. Pilot implementation of a home-care programme with chlamydia, gonorrhoea, hepatitis B, and syphilis self-sampling in HIV-positive men who have sex with men – Section: CONCLUSION
What Happens When STIs Go Untreated
Left untreated, many STIs cause damage that goes far beyond the initial infection site. Chlamydia and gonorrhea can travel upward through the reproductive tract in women, causing pelvic inflammatory disease, which in turn leads to scarring of the fallopian tubes. Most cases of tubal infertility are traced back to untreated STIs.14PubMed Central. Sexually transmitted diseases and infertility – Section: Abstract The same scarring also raises the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus, a potentially life-threatening condition.
Syphilis carries its own set of serious consequences. During pregnancy, untreated syphilis can cross the placenta and infect the fetus, leading to stillbirth, premature delivery, low birth weight, and congenital syphilis with long-term effects including deafness and neurological damage.15PubMed Central. Syphilis in HIV-Infected Mothers and Infants: Results from the NICHD/HPTN 040 Study – Section: Discussion 16PubMed Central. Syphilis Infection during pregnancy: fetal risks and clinical management In adults, late-stage syphilis can cause cardiovascular and neurological disease years after the initial infection. These outcomes are almost entirely preventable with early detection and treatment, which circles back to why screening matters so much.
Treatment and the Antibiotic Resistance Problem
Bacterial STIs are treated with antibiotics. Chlamydia typically responds well to doxycycline or azithromycin. Syphilis is still effectively treated with penicillin, which is remarkable given that the drug has been in use for this purpose since the 1940s. Trichomoniasis is treated with metronidazole or tinidazole. These treatments are inexpensive and highly effective when infections are caught early.
Gonorrhea is the outlier. Neisseria gonorrhoeae has progressively developed resistance to every class of antibiotic thrown at it over the decades. Ceftriaxone, an injectable antibiotic, is now the last remaining first-line treatment. Cases of high-level resistance to ceftriaxone have already been documented, raising concerns that gonorrhea could become effectively untreatable in some circumstances.17PubMed Central. Antibiotic resistance in Neisseria gonorrhoeae: origin, evolution, and lessons learned for the future – Section: Abstract This is one of the more pressing problems in infectious disease, and it means that anyone diagnosed with gonorrhea should follow up to confirm the infection has cleared.
Prevention Beyond Condoms
Condoms remain the most accessible and broadly effective barrier against STIs, but they are not the only tool available. The HPV vaccine has proven remarkably effective at preventing cervical cancer. Among women vaccinated before age 17, the rate of cervical cancer dropped by about 86% compared with unvaccinated women.18PubMed Central. Real-World Effectiveness of Human Papillomavirus Vaccination Against Cervical Cancer – Section: Abstract Vaccination is recommended for preteens of all sexes and remains beneficial for young adults who haven’t yet been vaccinated.
For HIV prevention, pre-exposure prophylaxis (PrEP) using antiretroviral medication has been transformative. Daily PrEP reduces HIV acquisition by roughly 44% to 86% depending on adherence levels, and event-driven dosing (taking pills around the time of sexual exposure) has shown a comparable reduction in clinical trials.19PubMed. Is event-driven PrEP dosing for HIV as effective as daily dosing? Adherence matters enormously: a demonstration study in West Africa found that HIV incidence was roughly four times higher among men using event-driven PrEP compared with daily PrEP, largely because adherence to the event-driven schedule was much lower in practice.20PubMed. Human Immunodeficiency Virus Seroconversion Among Men Who Have Sex With Men Who Use Event-Driven or Daily Oral Pre-Exposure Prophylaxis (CohMSM-PrEP) – Section: RESULTS
A newer strategy called doxycycline post-exposure prophylaxis, or doxy-PEP, involves taking a dose of doxycycline within 72 hours after unprotected sex to prevent bacterial STIs. A major trial found that this approach cut bacterial STI diagnoses by about two-thirds overall.21PubMed Central. Postexposure Doxycycline to Prevent Bacterial Sexually Transmitted Infections – Section: Results A meta-analysis confirmed strong reductions for syphilis (about 77% lower risk) and chlamydia (about 65% lower risk), but found no significant effect against gonorrhea, which fits with gonorrhea’s broader antibiotic resistance patterns.22Sexually Transmitted Infections. Efficacy of postexposure prophylaxis with doxycycline (Doxy-PEP) in reducing sexually transmitted infections: a systematic review and meta-analysis – Section: Abstract This approach is currently recommended for men who have sex with men and transgender women at elevated risk. Whether the widespread use of doxycycline for this purpose will accelerate antibiotic resistance more broadly is an open question that researchers are watching closely.
Treating Partners to Break the Cycle
One persistent challenge with STIs is reinfection. You can treat an infection successfully, but if your sexual partner still carries it, you’re likely to get it again. Expedited partner therapy (EPT) attempts to address this by allowing the diagnosed person to bring medication directly to their partner, bypassing the need for the partner to visit a clinic separately. Trials have generally shown that EPT increases the rate at which partners actually get treated, though evidence on whether it consistently reduces reinfection is mixed.23PubMed Central. Expedited partner therapy for sexually transmitted infections – Section: Abstract
Modeling research has suggested that even modest uptake of EPT could substantially reduce STI rates over time. One analysis estimated that providing EPT to just 20% of people diagnosed with gonorrhea or chlamydia could prevent over a quarter of expected infections across a ten-year period among men who have sex with men.24PubMed Central. Epidemiological impact of expedited partner therapy for men who have sex with men: A modeling study – Section: Discussion In practice, though, a study in adolescent women found that reinfection rates for chlamydia and trichomoniasis were not significantly different after EPT was introduced, and acceptance of EPT was inconsistent.25Sexually Transmitted Diseases. Expedited Partner Therapy in Female Adolescents: A Study of Acceptance and the Impact on Reinfection Rates – Section: Results The idea is sound, but real-world implementation is harder than the models predict.
Stigma as a Practical Barrier
Medical treatments for most STIs are effective and affordable. The harder problem is often getting people through the clinic door. Research conducted in the American South found that stigma operated through several channels: religious attitudes among health workers colored how they treated patients perceived as promiscuous, privacy fears discouraged men from visiting public health clinics, racial attitudes affected willingness to seek treatment, and fear of being labeled or “scarlet lettered” kept people from engaging with the health system entirely.26PubMed. Stigma as a barrier to treatment of sexually transmitted infection in the American deep south: issues of race, gender and poverty – Section: Abstract These barriers compound each other, particularly in communities where poverty limits options for discreet private care.
Stigma does not just delay treatment. It suppresses honest conversations between partners, discourages testing, and makes people less likely to disclose a diagnosis. Given how many infections are asymptomatic, every barrier to testing creates a longer chain of undetected transmission. Home self-collection kits, telehealth visits, and online ordering of test kits all help circumvent some of these barriers by removing the need to sit in a waiting room, but they do not eliminate the underlying stigma that shapes how people think about STIs.
The Vaginal Microbiome and STI Susceptibility
An area of growing research is how the community of bacteria naturally living in the genital tract influences whether someone who is exposed to an STI actually develops an infection. In women, a vaginal microbiome dominated by Lactobacillus species is generally associated with better reproductive health, while a shift toward more diverse anaerobic bacteria, sometimes manifesting as bacterial vaginosis, creates an environment that appears more permissive to STI acquisition.27PubMed Central. The role of the genital microbiota in the acquisition and pathogenesis of sexually transmitted infections – Section: Abstract
A large multicentre study of Chinese women of reproductive age found that most STIs, including those caused by Mycoplasma hominis, HPV, Ureaplasma urealyticum, chlamydia, and trichomoniasis, were associated with greater microbial diversity in the vagina, whereas vaginal yeast infection showed the opposite pattern.28Nature Communications. Vaginal microbiome and sexually-transmitted pathogens in Chinese reproductive-age women: a multicentre cross-sectional and longitudinal cohort study – Section: Association of STIs and co-infection with the baseline vaginal microbiome This does not mean a diverse microbiome “causes” STIs, but it does suggest that the local biological environment shapes vulnerability in ways we are only beginning to map. Whether interventions aimed at restoring Lactobacillus-dominant communities could reduce STI risk is still an open question, and no practical clinical recommendation has come from this research yet.
The Expanding Landscape of Sexually Transmitted Infections
The list of infections that can spread through sexual contact is growing. Traditional STIs like syphilis and gonorrhea are resurging in many countries after decades of decline. At the same time, infections not historically considered STIs are increasingly recognized as sexually transmissible under the right circumstances. These include certain fungal infections, enteric pathogens spread through oral-anal contact, and viral infections whose sexual transmission had previously been considered negligible.29Indian Journal of Dermatology, Venereology and Leprology. Newer and emerging sexually transmitted infections: A narrative review – Section: Abstract Mpox, which grabbed global headlines in 2022, is a recent example of a pathogen where close physical and sexual contact turned out to be a major transmission route even though the virus was not traditionally classified as an STI. Clinicians and public health officials are increasingly adopting the broader term “sexually transmissible infections” to capture this reality, acknowledging that sexual contact can facilitate the spread of pathogens well beyond the classic textbook list.