Syphilis spreads through direct contact with a syphilis sore during vaginal, anal, or oral sex. The bacterium responsible, called a spirochete, can penetrate intact mucous membranes or tiny breaks in the skin and reach the bloodstream within hours, making it a systemic infection almost immediately after exposure.
How the Bacterium Enters Your Body
The syphilis-causing organism is a corkscrew-shaped bacterium that moves quickly through tissue. It passes from one person to another when skin or a mucous membrane (the moist lining inside your mouth, genitals, or rectum) comes into direct contact with an infectious sore. These sores, called chancres, are painless and often go unnoticed. They appear at the spot where the bacterium first entered the body: the penis, vulva, vagina, anus, rectum, or lips and mouth.
Once the bacterium crosses the skin barrier, it spreads fast. Animal studies show spirochetes reach the lymphatic system within 30 minutes of initial contact, and the central nervous system is invaded early in the course of infection. This is why syphilis is considered a whole-body disease from the start, not just a localized sore.
Sexual Contact Is the Primary Route
The vast majority of syphilis cases are contracted through sexual activity. Any type of sex that involves direct contact with a sore can transmit the infection:
- Vaginal sex allows contact between genital mucous membranes and any sores present on either partner.
- Anal sex is a particularly efficient route because the rectal lining is thin and easily damaged, giving the bacterium easy entry.
- Oral sex can transmit syphilis in both directions. A sore on the genitals can infect the mouth, and a sore in or around the mouth can infect the genitals.
Condoms reduce the risk by preventing contact with sores, but they don’t eliminate it. Sores sometimes appear in areas a condom doesn’t cover, such as the scrotum, inner thighs, or around the anus. Contact with these exposed sores can still pass the infection.
Which Stages Are Most Contagious
Syphilis progresses through distinct stages, and the risk of spreading it depends heavily on where someone is in that progression. During the primary stage, one or more painless sores develop at the site of infection. These sores are teeming with bacteria and are highly contagious. The incubation period before the first sore appears ranges from 10 to 90 days, with an average of about 21 days after exposure.
If untreated, syphilis moves into the secondary stage. Skin rashes appear, often on the palms of the hands or soles of the feet, and sores can develop in the mouth, vagina, or anus. These secondary lesions are also infectious, sometimes even more so than the primary chancre because they can be widespread across the body. Someone with a rash on their hands or mouth-area sores could theoretically transmit the infection through close physical contact, though sexual contact remains the dominant route.
After the secondary stage, syphilis enters a latent (hidden) phase where there are no visible symptoms. During early latent syphilis (the first year or so), the infection can still potentially reactivate and become contagious again. Late latent syphilis, which can last years or even decades, is generally not transmissible to sexual partners, though it continues to damage the body internally if left untreated.
Transmission During Pregnancy
A pregnant person with syphilis can pass the infection to their baby through the placenta during pregnancy or during delivery. This is called congenital syphilis, and it can cause serious complications including stillbirth, low birth weight, and organ damage in the newborn. The risk is highest when the mother has an active, untreated infection, particularly during the primary or secondary stages. Routine prenatal screening catches most cases early enough for treatment to protect the baby.
Rare Non-Sexual Routes
Syphilis can theoretically spread through blood, though this is extremely uncommon in practice. Blood banks screen donations for syphilis, and the bacterium is fragile enough that it doesn’t survive standard blood storage conditions well. A study of over 5,500 kidney transplants in China found that donor-derived syphilis was rare, occurring only when donors had active, untreated infections. Modern transplant protocols include syphilis screening before donation and preventive treatment for recipients when a donor tests positive.
Sharing needles could also theoretically transmit syphilis if blood from an infected person enters another person’s body, though this route accounts for very few documented cases compared to sexual transmission.
What Doesn’t Spread Syphilis
The bacterium that causes syphilis is fragile and cannot survive long outside the human body. You cannot contract syphilis from toilet seats, doorknobs, swimming pools, shared clothing, or eating utensils. The CDC specifically notes that casual contact with objects like toilet seats cannot transmit the infection. Hugging, shaking hands, or sharing food also poses no risk. The bacterium requires the warm, moist environment of living tissue to survive, which is why direct contact with an active sore is essentially the only way it passes between people.
Current Risk in the United States
After years of rising case counts, syphilis rates in the U.S. have recently declined. Provisional CDC data show primary and secondary syphilis cases dropped 22% between 2023 and 2024, falling from about 53,000 cases to roughly 41,500. The rate per 100,000 people went from 15.8 to 12.2. While that decline is encouraging, tens of thousands of new cases each year means the infection remains common enough that anyone who is sexually active could encounter it, particularly people with multiple partners or those who don’t use barrier protection consistently.