Sexually transmitted infections are caused by at least five distinct categories of microorganism: bacteria, viruses, protozoan parasites, fungi, and ectoparasites such as lice and mites. The particular pathogen behind an infection determines almost everything about how it behaves, from whether it hides inside your cells or clings to the outside, to whether antibiotics can cure it or you carry it for life. Understanding what kind of organism you are dealing with is the first step toward making sense of symptoms, treatment, and prevention.
Bacteria That Cause STIs
Three of the most well-known STIs are bacterial: chlamydia, gonorrhea, and syphilis. Each uses a remarkably different strategy to survive in the human body, which is why they behave so differently despite all being treatable with antibiotics.
Chlamydia is caused by Chlamydia trachomatis, a bacterium that lives exclusively inside human cells. It cycles between two forms: a small, tough “elementary body” that can survive outside a cell long enough to infect a new one, and a larger “reticulate body” that reproduces inside a protective bubble called an inclusion once it has gotten inside a host cell. This intracellular lifestyle is why chlamydia often produces few or no symptoms for long stretches: the bacterium hides from the immune system inside the very cells it infects.1PubMed. Sphingolipids associate with the chlamydial nucleoid and mark developmental transitions in Chlamydia trachomatis
Gonorrhea, caused by Neisseria gonorrhoeae, takes a different approach. Rather than hiding inside cells, the gonococcus constantly reshuffles the proteins on its surface in a process called antigenic variation. It swaps gene segments in and out of the active slot for its pilus protein, producing so many surface variants that the immune system cannot keep up. This is why a person can be reinfected with gonorrhea repeatedly: even past exposure does not generate lasting immunity, because the bacterium’s surface is essentially a moving target.2PubMed Central. Restriction-modification systems are required for Neisseria gonorrhoeae pilin antigenic variation 3PubMed. Differential roles of homologous recombination pathways in Neisseria gonorrhoeae pilin antigenic variation, DNA transformation and DNA repair
Syphilis, caused by the spirochete Treponema pallidum, is arguably the most invasive of the three. This corkscrew-shaped bacterium is a remarkable traveler: it spreads rapidly through the bloodstream, squeezing between endothelial cells that line blood vessel walls without even disrupting them. It is the only species in its genus that can cross both the blood-brain barrier and the placental barrier, which is why untreated syphilis can eventually damage the brain and why it poses a serious risk during pregnancy.4PubMed Central. Treponema pallidum invades intercellular junctions of endothelial cell monolayers 5PubMed Central. Conservation of the Host-Interacting Proteins Tp0750 and Pallilysin among Treponemes and Restriction of Proteolytic Capacity to Treponema pallidum If untreated, T. pallidum can establish latency lasting decades before progressing to severe tissue destruction.6PubMed. Syphilis Pathogenesis: Host Interactions, Immune Evasion, and Persistence of Treponema pallidum
Mycoplasma genitalium, the Newer Name on the List
Not all bacterial STIs have been recognized for as long as the classic trio. Mycoplasma genitalium was identified in the early 1980s and has since emerged as an established cause of non-gonococcal urethritis in men and pelvic inflammatory disease in women.7PubMed Central. Mycoplasma genitalium: an emerging sexually transmitted pathogen What makes it unusual among bacteria is that it lacks a rigid cell wall entirely. Instead, it relies on a specialized adhesion complex on its outer membrane to latch onto host cells and glide along them.8PubMed. Structural characterization of the NAP; the major adhesion complex of the human pathogen Mycoplasma genitalium Because standard testing panels have only recently begun screening for it, many infections go undiagnosed and are treated with broad-spectrum antibiotics that may not work well against it.
Viruses That Cause STIs
Viral STIs are caused by fundamentally different organisms than bacterial ones, and that difference shapes nearly every aspect of the infection. Bacteria are living cells that can reproduce on their own and can usually be killed with the right antibiotic. Viruses hijack your own cells’ machinery to make copies of themselves and, in many cases, cannot be eliminated from the body once they establish themselves.
Human papillomavirus (HPV) is the most common sexually transmitted virus. Most infections clear on their own, but high-risk strains produce two proteins, E6 and E7, that interfere with the cell’s built-in tumor-suppression systems. Over time, this interference can push cells toward uncontrolled growth, and high-risk HPV is responsible for roughly 5% of all human cancers worldwide.9Journal of Biological Chemistry. Direct interaction between human papillomavirus E6 and E7 oncoproteins 10PubMed Central. High-Risk Human Papillomaviral Oncogenes E6 and E7 Target Key Cellular Pathways to Achieve Oncogenesis Vaccination against the most dangerous strains has dramatically reduced infection rates where it is widely used.
Herpes simplex virus (HSV) uses a different trick: latency. After the initial infection in surface skin or mucous membranes, the virus travels along nerve fibers and parks its genetic material in the nuclei of sensory neurons, where the immune system largely cannot reach it. Periodically, the virus reactivates, travels back down the nerve, and produces new infectious particles at the skin surface, which is why herpes outbreaks recur and why a person can transmit the virus even between visible episodes.11PubMed Central. A cultured affair: HSV latency and reactivation in neurons 12PubMed Central. Strength in diversity: Understanding the pathways to herpes simplex virus reactivation
HIV targets a different cell type altogether. The virus uses chemokine receptors, particularly CCR5 and CXCR4, as co-receptors to enter CD4+ immune cells. Early in infection, viral strains tend to use CCR5 for entry, while strains isolated later in the course of disease shift toward using CXCR4.13PubMed. The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes By killing the immune cells it infects, HIV progressively weakens the immune system, which is what makes it so dangerous over time without treatment. Modern antiretroviral therapy can suppress the virus to undetectable levels, but it cannot clear the latent reservoir entirely.
Hepatitis B (HBV) is another sexually transmitted virus with a chronic component. The virus infects liver cells, and its replication is influenced by host hormones, with androgen receptor stimulation linked to increased production of viral proteins and DNA. This hormonal interaction helps explain some of the sex-based differences in chronic HBV outcomes.14PubMed Central. Sexual Dimorphism in Chronic Hepatitis B Virus (HBV) Infection: Evidence to Inform Elimination Efforts
Parasites, Lice, and Mites
The “and more” in the list of STI causes includes several organisms that are neither bacteria nor viruses. Trichomonas vaginalis is a single-celled protozoan parasite and the cause of trichomoniasis, one of the most common curable STIs globally. Unlike chlamydia, trichomoniasis is an extracellular infection: the parasite sticks to the surface of urogenital epithelial cells rather than entering them. Once attached, it triggers inflammation, produces cytotoxic damage to the tissue, and can provoke an immune response that causes noticeable discharge and irritation.15PubMed Central. Trichomonas vaginalis contact-dependent cytolysis of epithelial cells Research on prostate epithelial cells has shown that the parasite’s adhesion increases steadily over hours and leads to inflammatory cytokine production.16PubMed Central. Interaction between Trichomonas vaginalis and the Prostate Epithelium Interestingly, strain-to-strain variation in T. vaginalis is enormous: studies have found a 12-fold range in how strongly different strains adhere to ectocervical cells and a 45-fold range in adherence to prostate cells, which may partly explain why some infections are nearly asymptomatic while others cause substantial discomfort.15PubMed Central. Trichomonas vaginalis contact-dependent cytolysis of epithelial cells
Then there are the ectoparasites: pubic lice (Phthiris pubis) and the scabies mite (Sarcoptes scabiei). These organisms live on or within the skin and require the sustained close physical contact that sexual activity provides. Condoms do not protect against either, because transmission depends on skin-to-skin proximity rather than exchange of bodily fluids. Scabies mites burrow into the outer layer of skin and can also spread through prolonged non-sexual contact such as hand-holding, though acquiring them from shared bedding or clothing is uncommon.17PubMed Central. Scabies, Bedbug, and Body Lice Infestations: A Review 18Medicine. Lice and scabies
Fungi and the Line Between Infection and Imbalance
Candida albicans, the yeast behind most vaginal yeast infections, is technically not classified as a classic STI in most medical guidelines. It normally lives in the vaginal environment without causing problems. The shift from harmless resident to disease-causing agent happens when conditions change: antibiotic use, hormonal shifts, immune suppression, or disruption of the vaginal microbial balance can all allow Candida to overgrow. Up to three-quarters of women will experience at least one episode of vulvovaginal candidiasis in their lifetime, and roughly 5 to 10% develop recurrent episodes.19PubMed Central. A Comprehensive Overview of Candida albicans as the Leading Pathogen in Vulvovaginal Candidiasis The organism uses virulence tactics like switching from a round yeast form to invasive filaments, forming biofilms, and evading immune detection. Whether sexual transmission contributes meaningfully to yeast infections remains debated, but sexual activity can alter the vaginal environment enough to tip the balance.
How the Vaginal Microbiome Influences Susceptibility
The composition of a person’s genital microbiome turns out to be a major factor in whether exposure to a pathogen leads to an actual infection. A healthy cervicovaginal environment is dominated by Lactobacillus species that maintain an acidic pH, produce antimicrobial compounds, and shape the local immune response. Growing evidence suggests these bacteria can help guard against viral STIs including HPV, HIV, and HSV.20PubMed Central. Protective Mechanisms of Vaginal Lactobacilli against Sexually Transmitted Viral Infections
When this protective community is disrupted, the result is often bacterial vaginosis (BV), a condition in which the lactobacilli are replaced by a diverse mix of anaerobic bacteria. BV is not itself an STI in the traditional sense, but it is an independent risk factor for acquiring STIs, developing pelvic inflammatory disease, and experiencing preterm birth.21PubMed Central. Vaginal microbiome and sexually transmitted infections: an epidemiologic perspective 22PubMed Central. Vaginal microbiota and the potential of Lactobacillus derivatives in maintaining vaginal health In other words, the environment a pathogen lands in matters almost as much as the pathogen itself.
How Pathogens Actually Get In
Sexual transmission is not simply about passing organisms from one person to another. The physical mechanics of sex create conditions that make infection more likely. Mucosal surfaces in the genital and anal areas are thinner and more vulnerable than regular skin, and the friction of sexual contact can produce microabrasions, tiny breaks in tissue that strip away upper protective layers. These micro-injuries expose deeper cell surfaces that pathogens can latch onto. For viruses in particular, disrupting the tight junctions between cells can expose receptors on the sides and undersides of cells that are normally hidden.23Nature Communications. Highly efficient anogenital transmission of clade Ia monkeypox virus associated with increased shedding This mechanism has been documented for HIV, HSV, and mpox, and likely applies broadly across sexually transmitted pathogens.
How STIs Help Each Other
One of the more unsettling realities of STIs is that they do not exist in isolation. Having one infection often makes it easier to acquire another, creating what researchers call synergistic co-infections. The best-studied example involves HSV-2 and HIV. Tissue infected by herpes or syphilis contains a higher concentration of CCR5-expressing T cells, which are exactly the cells HIV targets for entry. This means that active herpes sores or syphilitic lesions essentially create a welcoming environment for HIV at the tissue level.24PubMed Central. Herpes simplex virus type 2 and syphilis infections with HIV: an evolving synergy in transmission and prevention Any STI that causes inflammation or breaks in mucosal tissue raises the odds of acquiring other infections, which is one reason that comprehensive STI screening matters even for infections that seem mild on their own.
Passing Infections Without Sex
The term “sexually transmitted” can be misleading. Many of these same pathogens also spread through non-sexual routes, particularly from parent to child. Vertical transmission can occur across the placenta during pregnancy or during passage through the birth canal. The major mechanism differs by pathogen: syphilis primarily crosses the placenta during gestation, while HIV, hepatitis B, HSV, gonorrhea, and chlamydia are more often transmitted during delivery.25PubMed. Vertical trasmission of human immunodeficiency virus (HIV) and other sexually transmitted infections (STI) This is why prenatal STI screening is standard in most healthcare systems: catching an infection in the parent creates an opportunity to treat it or plan the delivery to minimize risk to the baby.
Why Some People Are More Susceptible
Exposure to a pathogen does not guarantee infection, and the reasons go beyond behavioral factors like condom use. Your genetic makeup plays a measurable role. Variations in genes that control immune responses, cell-surface receptors, and inflammatory signaling can make one person more vulnerable to a given STI while another person exposed to the same pathogen clears it without trouble. These polymorphisms can affect how strongly your body reacts to bacterial, viral, or parasitic invaders, influencing both whether an infection takes hold and how quickly it progresses.26PubMed Central. The association of host genes with specific sexually transmitted infections Research in this area is still relatively early, but it reinforces an important point: contracting an STI is not purely a matter of behavior or bad luck. Biology tilts the playing field in ways we are only beginning to map.
The Antibiotic Resistance Problem
For bacterial STIs, the assumption that “at least they are curable” is becoming less reliable. Neisseria gonorrhoeae has developed resistance to every class of antibiotic that has been used against it over the decades: sulfonamides, penicillins, tetracyclines, fluoroquinolones, and now extended-spectrum cephalosporins. A strain displaying high-level resistance to ceftriaxone, long considered the last reliable first-line treatment, was identified in Japan and raised alarm worldwide.27PubMed Central. Antibiotic resistance in Neisseria gonorrhoeae: origin, evolution, and lessons learned for the future The gonococcus’s ability to acquire and share resistance genes is tied to the same genetic flexibility that makes its surface proteins so variable. Mycoplasma genitalium is also accumulating resistance, particularly to the macrolide antibiotics most commonly used against it. These trends mean that treatment guidelines are updated frequently, and what worked a few years ago may no longer be the first choice.
How Different Pathogens Are Detected
The type of organism behind an STI also determines how it is diagnosed. For decades, gonorrhea diagnosis relied on growing the bacterium in a lab culture, and chlamydia detection depended on enzyme-based assays. Both methods have significant sensitivity limitations: culture catches only about 70% of gonorrhea infections, and the traditional chlamydia assay catches only around 42%. Nucleic acid amplification tests, which detect tiny amounts of pathogen DNA, have largely replaced these older methods because they catch a much higher proportion of infections, often above 90%, with very few false positives.28PubMed. Detection of Chlamydia trachomatis and Neisseria gonorrhoeae by enzyme immunoassay, culture, and three nucleic acid amplification tests For viral infections like HIV and hepatitis B, blood-based tests looking for antibodies or viral genetic material are standard. Herpes is sometimes diagnosed by swabbing an active lesion, but blood tests for HSV antibodies exist as well, though their usefulness in people without symptoms is debated. Trichomoniasis can be detected by microscopy, culture, or molecular testing, with molecular methods again being the most sensitive. The practical takeaway is that testing technology matters: a negative result on an older, less sensitive test does not carry the same weight as a negative on a modern molecular assay, and if you have reason for concern, it is worth asking your provider which test was used.