Syphilis spreads overwhelmingly through sexual contact, but it is not exclusively a sexually transmitted infection. The bacterium responsible, Treponema pallidum, can pass from one person to another through several non-sexual routes, including from mother to child during pregnancy, through blood transfusions, organ transplants, and even kissing when an active sore is present. These non-sexual routes are uncommon compared to sexual transmission, but they are well documented and worth understanding.
Why Syphilis Is Almost Always Sexual
Treponema pallidum is an unusually fragile organism. It cannot survive for long outside the human body and is extremely sensitive to changes in temperature, drying, and oxygen exposure.1PubMed Central. Biological basis for syphilis Scientists have never been able to grow it continuously in a laboratory dish, which tells you something about how dependent it is on the warm, moist conditions inside a living host. This fragility is why you cannot catch syphilis from toilet seats, swimming pools, shared utensils, or doorknobs. The bacterium simply does not persist on surfaces.
What syphilis needs to spread is direct contact between an infected person’s active sore or mucous membrane and a break in another person’s skin or mucous membrane. Sexual contact provides that opportunity efficiently, which is why it accounts for the vast majority of cases. But any form of intimate physical contact that meets the same biological requirements, meaning moist tissue touching an active lesion, can theoretically allow transmission. That distinction matters when you are trying to understand the exceptions.
Congenital Syphilis and Transmission Through Breastfeeding
The most significant non-sexual route of syphilis transmission is from a pregnant person to their baby. When a mother has untreated syphilis during pregnancy, the bacterium can cross the placenta and infect the fetus. This is called congenital syphilis, and it can cause miscarriage, stillbirth, or severe health problems in the newborn including bone abnormalities, anemia, and organ damage. Rates of congenital syphilis have been rising in many countries, making this a growing public health concern.
A less well-known variation involves postnatal transmission through breastfeeding. In one documented case, a baby just under two months old was diagnosed with congenital syphilis even though the biological mother tested negative for the infection. The infant had been breastfed by a relative who was later found to be syphilis-positive.2Child’s Health. Breastfeeding-associated congenital syphilis in an infant of a seronegative mother: a case report This case highlights that T. pallidum can enter a baby’s body through contact with an infected person’s breast lesions during nursing. It also makes diagnosis harder, because clinicians naturally assume syphilis in a newborn came from the mother.
Kissing and Direct Skin Contact
Syphilis can be transmitted through deep kissing when one person has an active sore on or around the mouth. Primary syphilis produces a painless ulcer called a chancre, and when that chancre happens to appear on the lip or inside the mouth, kissing becomes a realistic transmission route. A case report published in Medicine described a woman who developed a syphilitic chancre on her lip after kissing her husband, who had acquired oral syphilis from someone else through oral sex. Neither she nor her husband reported any orogenital contact between themselves.3PubMed Central. Syphilitic Chancre of the Lips Transmitted by Kissing: A Case Report and Review of the Literature
This kind of case is uncommon, but it illustrates an important point: what matters for syphilis transmission is not the sexual nature of the contact, but whether an active infectious lesion comes into direct contact with another person’s mucous membrane or broken skin. Mouth-to-mouth kissing can meet that condition. Casual skin contact, like a handshake or a hug, does not, because the bacterium cannot penetrate intact, dry skin.
Non-Sexual Transmission in Children
Children can acquire syphilis through non-sexual contact, and this is a fact with uncomfortable clinical implications. When a child tests positive for syphilis, the immediate concern is often sexual abuse, and that suspicion is frequently warranted. But close and repetitive contact with the mucosal or skin lesions of a household member with active syphilis can also explain the infection.4The Pediatric Infectious Disease Journal. Acquired Syphilis by Nonsexual Contact in Childhood This might happen when a parent or caregiver with an oral syphilis sore shares food, cups, or kisses the child frequently.
The clinical challenge is distinguishing between non-sexual household transmission and abuse. Both produce the same laboratory findings. Clinicians are trained to consider the full picture: the location of any lesions on the child, the child’s age, the household contacts’ infection status, and whether there are other signs consistent with abuse. The existence of non-sexual transmission in children does not eliminate the need to investigate, but it does mean that a positive syphilis test in a child is not automatic proof of sexual contact.
Blood Transfusions
Syphilis was once a recognized risk of blood transfusion, and in parts of the world where screening is less comprehensive, it still is. A study at a Nigerian teaching hospital found that 8% of blood donor samples tested positive for T. pallidum, with significantly higher rates among paid commercial donors compared to voluntary ones. The researchers estimated that hundreds of cases per year of transfusion-transmitted syphilis could occur at that single hospital based on its rate of blood utilization.5PubMed Central. Risk of transfusion-transmitted syphilis in a tertiary hospital in Nigeria
In countries with well-established blood banking systems, like the United States, Canada, and most of Europe, every donated unit of blood is screened for syphilis antibodies. Positive units are discarded. The fragility of T. pallidum also works in favor of blood safety: the bacterium does not survive well during standard blood storage at refrigerated temperatures. Between screening and refrigeration, the risk of getting syphilis from a blood transfusion in a high-income country is effectively negligible today. In settings with less rigorous screening infrastructure, the risk remains real.
Organ Transplantation
Organ transplantation presents a different risk profile. Unlike blood, transplanted organs come from donors who sometimes have active or past syphilis infections, and the organs are transplanted quickly rather than being stored for days at cold temperatures. As syphilis rates rise in the general population, transplant centers are seeing more donor organs from syphilis-positive individuals.
A U.S.-based retrospective study followed 25 syphilis-negative recipients who received organs from 11 donors with reactive syphilis tests. Despite receiving preventive antibiotic therapy shortly after surgery, three of those 25 recipients showed seroconversion, meaning their blood tests turned positive for syphilis antibodies at three months. However, confirmatory testing was negative in all three cases, suggesting the positive result may have reflected passive transfer of antibodies from the donor rather than active infection.6PubMed. Serologic follow-up of solid organ transplant recipients who received organs from donors with reactive syphilis tests: A retrospective cohort study
A larger study from China examined over 5,500 kidney transplant recipients and found confirmed donor-derived syphilis in seven cases, a rate of about 0.1%.7Emerging Infectious Diseases. Risk for Donor-Derived Syphilis after Kidney Transplantation, China, 2007–2022 The risk was concentrated among recipients who received organs from donors with both positive screening and confirmatory tests. These numbers suggest that donor-derived syphilis transmission is rare but not zero, and the standard practice of treating recipients with antibiotics after transplant from a syphilis-positive donor appears to reduce the risk substantially.
Tattooing and Needle-Related Risks
Tattooing has been linked to syphilis transmission in the medical literature, though the evidence is older and largely based on case reports rather than large studies. A review of infectious complications associated with tattoos identified Treponema pallidum among the organisms that have been transmitted through the procedure.8Clinical Infectious Diseases. Infectious Complications of Tattoos The mechanism would involve contaminated needles or ink being reused between clients without proper sterilization.
In licensed, regulated tattoo shops in most developed countries, single-use needles and sterile practices have made this risk extremely low. The concern is more relevant in informal settings, home tattooing, or in regions where infection control standards are inconsistent. Shared injection drug equipment carries a similar theoretical risk, since any break in the skin contaminated with infected blood could introduce the organism, though this route is rarely documented compared to hepatitis or HIV transmission from needle sharing.
Why a Positive Test Does Not Always Mean Infection
If you have never had sexual contact with anyone who has syphilis and you receive a positive syphilis test result, there is another explanation worth considering before non-sexual transmission: a false positive. Syphilis testing, especially the initial screening step, is known for producing false positives in people who do not actually have the infection. Discordant test results, where the initial screening test is positive but the confirmatory test is negative, are typically considered biological false positives and can be triggered by other medical conditions.9PubMed Central. Frequency and Characteristics of Biological False-Positive Test Results for Syphilis Reported in Florida and New York City, USA, 2013 to 2017
Conditions that can cause false-positive syphilis screening results include autoimmune diseases like lupus, certain viral infections, pregnancy, liver disease, and recent vaccinations. In children, false positives present a particular diagnostic headache because they can trigger unnecessary child-protection investigations. A study examining false-positive syphilis results in children found that younger age and certain blood-clotting abnormalities were associated with higher rates of false reactivity.10PubMed Central. Factors associated with false positive results in serological testing for syphilis using EIA among children
The practical takeaway is that a single reactive syphilis screening test should always be followed by confirmatory testing. If the confirmatory test is negative, the result is almost certainly a false positive. If you are told you have syphilis and you have no plausible exposure history, ask your doctor which tests were run and whether the confirmatory test was positive.
Syphilis as “The Great Imitator”
Part of the reason non-sexual syphilis transmission gets attention is that syphilis itself is famously difficult to diagnose on clinical appearance alone. It has earned the nickname “the great imitator” because its skin lesions, rashes, and systemic symptoms can look like dozens of other conditions.11PubMed Central. The great imitator revealed: syphilis A rash from secondary syphilis can be mistaken for an allergic reaction, psoriasis, or a viral illness. A painless genital ulcer can be confused with herpes or a harmless skin condition.
This mimicry means that some cases attributed to non-sexual routes might actually reflect unrecognized sexual exposure. A person who develops a lesion after kissing someone might actually have had earlier sexual exposure they did not realize was risky. The reverse also happens: a person correctly infected through non-sexual contact might be incorrectly told their infection must have come from sex because that is the default assumption. Clinicians who are aware of the non-sexual routes are better positioned to take an accurate history and avoid misattributing the source of infection.
Related Infections That Spread Without Sex
There is a group of diseases caused by bacteria so closely related to the syphilis bacterium that standard blood tests cannot distinguish them. These are the endemic treponematoses: yaws, bejel (also called endemic syphilis), and pinta. Each is caused by a subspecies of Treponema pallidum or a closely related species, and all spread through non-sexual skin-to-skin contact, primarily among children in tropical or arid regions.12PubMed Central. The endemic treponematoses
Yaws, caused by T. pallidum subspecies pertenue, is the most common of these. It produces skin ulcers and can eventually damage bone and cartilage if untreated. It spreads in humid tropical regions through direct skin contact with active lesions, predominantly affecting children under 15.13PubMed. Yaws Bejel, caused by T. pallidum subspecies endemicum, spreads similarly but tends to occur in hot, arid environments and is sometimes transmitted through shared eating utensils or cups.14Oxford Textbook of Medicine. Nonvenereal endemic treponematoses: yaws, endemic syphilis (bejel), and pinta
The genetic similarity between these organisms is striking. Whole genome comparisons show that yaws and syphilis treponemes differ in less than 0.2% of their genome sequence.15PLoS Neglected Tropical Diseases. Whole Genome Sequences of Three Treponema pallidum ssp. pertenue Strains: Yaws and Syphilis Treponemes Differ in Less than 0.2% of the Genome Sequence Despite that near-identical DNA, the diseases they cause behave quite differently. Venereal syphilis can invade the nervous system and cardiovascular system and is transmitted sexually, while yaws and bejel primarily affect skin and bone and spread through casual contact. Genomic analyses have identified differences in pathogenicity islands and other gene regions that may help explain why such genetically similar organisms cause such different disease patterns.16PubMed Central. The pan-genome of Treponema pallidum reveals differences in genome plasticity between subspecies related to venereal and non-venereal syphilis
The existence of these endemic treponematoses matters for two practical reasons. First, anyone who has had yaws or bejel will test positive on standard syphilis blood tests, because the antibodies are indistinguishable. A person who grew up in a yaws-endemic region and later moves to a country where venereal syphilis is common may be told they have syphilis when they actually had a childhood case of yaws. Second, these diseases prove that the treponema family of bacteria is fully capable of non-sexual transmission under the right conditions, which makes it less surprising that venereal syphilis occasionally does the same.
Global Efforts to Eliminate Non-Venereal Treponematoses
Yaws was nearly eradicated in the mid-twentieth century through mass penicillin campaigns, but it bounced back after efforts slowed. The World Health Organization has renewed its push to eliminate the disease, this time using oral azithromycin rather than injectable penicillin. A randomized trial in Papua New Guinea compared standard community-based treatment to an experimental approach of three rounds of mass drug administration with azithromycin. The experimental approach reduced the prevalence of active yaws dramatically, to about 0.04% from much higher baseline levels.17PubMed Central. A Trial of Three Rounds of Mass Drug Administration with Azithromycin for Yaws
Eliminating yaws and bejel matters not just for the communities where they cause suffering, but also because their serological cross-reactivity with venereal syphilis complicates global syphilis surveillance and diagnosis. As long as these infections persist, health workers in endemic regions face the challenge of distinguishing between a sexually transmitted disease and a childhood skin infection, using tests that cannot tell the difference. If yaws can be eliminated, one source of diagnostic confusion about non-sexual syphilis acquisition would disappear with it.