Syphilis spreads almost exclusively through direct contact with an active syphilis sore or rash during vaginal, anal, or oral sex. The bacterium responsible, Treponema pallidum, can also pass from a pregnant person to their developing baby. Beyond those two routes, transmission is rare. But the details of how and when the bacterium moves between people are more nuanced than most health summaries let on, and some of the most persistent beliefs about syphilis transmission are flat-out wrong.
How the Bacterium Gets In
Syphilis is caused by a corkscrew-shaped bacterium called Treponema pallidum. Unlike many other pathogens, it does not just sit on a surface and wait to be picked up. It actively drills into tissue. Spirochetes like T. pallidum have internal flagella that rotate inside the cell body, causing the entire organism to spin and undulate. That motion generates the force the bacterium needs to bore through mucous membranes and into deeper tissue layers.1PubMed Central. Flagellar motility of the pathogenic spirochetes This is why syphilis does not need a large open wound to establish an infection. Microscopic abrasions in the lining of the mouth, genitals, or rectum are enough.
Once it penetrates the surface, the bacterium enters the lymphatic system and bloodstream relatively quickly and begins to disseminate throughout the body.2PubMed Central. Optic Neuritis As the Initial Presentation of Syphilis That rapid spread is why syphilis can eventually affect the brain, eyes, heart, and bones if left untreated, even though the initial infection may look like nothing more than a painless sore at the point of contact.
Sexual Transmission
The vast majority of syphilis cases are acquired through sexual contact. Vaginal sex, anal sex, and oral sex can all transmit the infection. The key factor is direct skin-to-skin or mucous-membrane contact with a syphilis lesion, which can be a chancre (the firm, painless sore of primary syphilis) or the rash and mucous patches of secondary syphilis. The sore does not have to be on the genitals. Chancres can appear on the lips, tongue, throat, anus, or rectum, and they are often painless enough that the infected person does not notice them.
Anal sex carries a particularly high per-act risk because the rectal lining is thin and prone to micro-tears. Oral sex is lower risk but far from zero, especially when the infected partner has a sore in the mouth or on the lips. Because chancres are painless and can hide inside the vagina, rectum, or throat, many people with active primary syphilis have no idea they are infectious.
Condoms reduce the risk but do not eliminate it. A sore outside the area covered by a condom can still make contact with a partner’s skin or mucous membranes. That said, consistent condom use remains one of the most practical tools for lowering the chance of transmission during any given sexual encounter.
Transmission Without Visible Symptoms
One of the most important things to understand about syphilis is that you do not need to see a sore to catch it, and the person who gives it to you may not know they are infected. A systematic review of studies examining transmission from people without visible symptoms found direct and indirect evidence that individuals with primary, secondary, and early latent syphilis can pass the infection along before noticing any lesions. Modern contact-tracing studies show that sexual partners of asymptomatic people with early syphilis frequently test positive, with transmission rates ranging from roughly 10% to over 60% depending on the stage of infection and how long the partnership lasted.3PubMed Central. What Is the Evidence for the Sexual Transmission of Syphilis From Asymptomatic Persons?
This matters for a practical reason. Many people assume that syphilis is only contagious when a sore is visible, so they believe they can rely on visual inspection of a partner’s genitals. That assumption is wrong. The chancre of primary syphilis can be internal, hidden inside the cervix or rectum. And in early latent syphilis, there are no symptoms at all, yet the person is still infectious. Routine screening, especially for people with multiple sexual partners, is the only reliable way to catch infections during these silent periods.
From Mother to Baby
The second major transmission route is from a pregnant person to their developing fetus. Treponema pallidum can cross the placenta and infect the baby at any point during pregnancy, and it can also be transmitted through contact with a maternal sore during delivery.4PubMed Central. Congenital Syphilis-An Illustrative Review The consequences of congenital syphilis range from no visible symptoms at birth to stillbirth, low birth weight, liver and spleen enlargement, bone abnormalities, rash, and infection of the central nervous system.5PubMed. Research progress on the mechanism of Treponema pallidum breaking through placental barrier
Congenital syphilis remains a significant cause of fetal death worldwide, and it is almost entirely preventable. Treating an infected pregnant person with penicillin before delivery can stop the bacterium from reaching the fetus. The reason cases continue to rise in many countries is not a failure of medicine but a failure of screening. Prenatal syphilis testing is standard in most healthcare systems, but people who receive late or inconsistent prenatal care are far more likely to have untreated infections at the time of delivery.6Obstetric Anesthesia Digest. Syphilis Complicating Pregnancy and Congenital Syphilis
What Does NOT Spread Syphilis
There is a surprising amount of misinformation circulating about syphilis transmission, particularly on social media. A study analyzing short-form videos about syphilis on YouTube found multiple videos explicitly stating or implying that syphilis could spread through casual contact, including sharing drinking glasses or kissing someone on the cheek.7PubMed Central. Hidden misinformation in YouTube short videos on syphilis: A mixed-methods study These claims are not supported by evidence.
The biology of T. pallidum itself explains why casual transmission is essentially impossible. The bacterium is extraordinarily fragile outside the human body. Laboratory research shows that T. pallidum survives best in very low oxygen concentrations, thriving under microaerophilic conditions that approximate the environment inside living tissue.8Sexually Transmitted Infections. Optimum concentration of dissolved oxygen for the survival of virulent Treponema pallidum under conditions of low oxidation-reduction potential Exposure to normal atmospheric oxygen, drying, temperature changes, or common disinfectants kills it rapidly. That is why you cannot get syphilis from toilet seats, doorknobs, swimming pools, shared utensils, or clothing. The organism simply cannot survive on those surfaces long enough to infect anyone.
Kissing is a gray area that deserves a careful answer. A quick social kiss on the cheek or lips poses no meaningful risk. Deep kissing with someone who has an active syphilis sore in their mouth is theoretically a transmission route, because it involves direct mucous-membrane contact with an infectious lesion. In practice, oral chancres are uncommon enough that this is not a major driver of syphilis spread, but it is not zero risk either. The distinction comes down to whether there is direct contact with an active sore, not whether saliva was exchanged.
Blood transfusion and organ donation are also potential but extremely rare transmission routes in countries with modern screening protocols. Donated blood is tested for syphilis, and the bacterium’s fragility means it does not survive the storage conditions used for banked blood. These routes were more relevant historically than they are today.
The Syphilis-HIV Connection
Having syphilis makes you substantially more vulnerable to HIV, and this relationship has shaped public health strategies around both infections for decades. The syphilitic ulcers that form during primary and secondary syphilis break the protective lining of the genitals, rectum, or mouth, creating a direct entry point for HIV. But the connection goes deeper than a simple physical wound. The immune response at a syphilis sore actually concentrates the very cells that HIV targets. Macrophages and CD4-positive T lymphocytes flood the area around a T. pallidum infection site, and these are exactly the cells HIV attaches to and invades.9PubMed Central. Effect of syphilis infection on HIV acquisition: a systematic review and meta-analysis
In other words, syphilis does not just open a door for HIV. It rolls out a welcome mat by assembling a dense cluster of HIV-susceptible immune cells right at the site of the breach. This is why syphilis outbreaks in a community are often followed by increases in new HIV infections. The resurgence of syphilis among men who have sex with men in the 2000s, linked in part to changes in sexual networking and substance use, has been closely tracked alongside HIV surveillance for this reason.10PubMed Central. Evolution of the syphilis epidemic among men who have sex with men
Getting Syphilis Again After Treatment
A common misconception is that once you have had syphilis and been treated, your immune system will protect you from future infections. It will not. Syphilis does not produce reliable lasting immunity. A large retrospective study in Antwerp found that the majority of syphilis episodes recorded over two decades occurred in people who had already had syphilis before, with repeat infections accounting for about 61% of all episodes.11PubMed. Syphilis reinfections pose problems for syphilis diagnosis in Antwerp, Belgium – 1992 to 2012
The immunology of reinfection is genuinely unusual. One study tracking individuals through multiple syphilis episodes found evidence that each round of infection may produce a slightly stronger immune response, suggesting a cumulative but incomplete buildup of partial immunity.12PubMed Central. Syphilis reinfection is associated with an attenuated immune profile in the same individual: a prospective observational cohort study But “slightly stronger” is a long way from “protective.” Reinfections continue to happen even after multiple prior episodes. And reinfections in people living with HIV are especially likely to be asymptomatic, which makes them easy to miss without regular screening.13PubMed Central. Syphilis Infections, Reinfections and Serological Response in a Large Italian Sexually Transmitted Disease Centre
Reinfection also creates diagnostic headaches. Standard blood tests for syphilis look for antibodies, and someone who has been treated for syphilis may retain antibodies from the previous infection. Distinguishing a new infection from old antibodies requires comparing current results to past results and tracking changes over time. This is one reason why having a consistent healthcare provider who knows your testing history is valuable if you are at ongoing risk.
Nonvenereal Treponemal Diseases
Syphilis is not the only disease caused by a treponeme. Yaws, endemic syphilis (also called bejel), and pinta are caused by closely related subspecies of Treponema and produce skin and bone lesions that can look strikingly similar to venereal syphilis. The critical difference is in how they spread. These nonvenereal treponematoses are transmitted through casual skin-to-skin contact, often among children in tropical regions, not through sex.14PubMed. Nonvenereal treponematoses: yaws, endemic syphilis, and pinta The organisms are so similar that standard syphilis blood tests cannot distinguish between them, which can cause confusion when someone from a region where yaws is endemic moves to a country where venereal syphilis is the only form clinicians expect to see.
These related diseases also complicate the evolutionary story of syphilis itself. Some researchers have argued that venereal syphilis may have evolved from a nonvenereal ancestor as human populations migrated and changed their patterns of skin contact and clothing use. That debate is not fully settled, but it underscores a key point about T. pallidum: this organism’s transmission route is a product of its biological niche, not an inherent property of all treponemes.
Doxycycline as an Emerging Prevention Tool
For people at high ongoing risk of syphilis, a new prevention strategy has gained significant traction in recent years. Doxycycline, a common antibiotic, taken either before or after sexual exposure, has shown striking effectiveness in reducing syphilis infections. An updated meta-analysis of randomized and non-randomized trials, primarily involving men who have sex with men and transgender women, found that doxycycline reduced syphilis acquisition by about 78% in randomized trial data.15PubMed. Doxycycline prophylaxis is effective as pre-exposure and post-exposure regimens in the prevention of sexually transmitted infections: an updated systematic review and meta-analysis
That is a large effect, and health departments in several cities have already begun incorporating doxycycline post-exposure prophylaxis into their guidelines for people with recurrent sexually transmitted infections. The approach is not without controversy. Widespread antibiotic use raises legitimate concerns about driving antimicrobial resistance, and the evidence base is still heavily concentrated in specific populations. Doxycycline prophylaxis has not been well studied in cisgender women, for example, partly because early trial data suggested lower effectiveness in that group, though the reasons remain unclear.
Doxycycline prophylaxis also does nothing to prevent HIV or viral infections like herpes. It targets bacterial sexually transmitted infections specifically. For someone trying to assemble a comprehensive prevention strategy, it is one tool among several, including condoms, regular screening, prompt treatment of partners, and HIV pre-exposure prophylaxis for those at risk of both infections. No single intervention eliminates the risk of syphilis entirely, but combining them pushes the odds substantially in your favor.