Syphilis spreads through direct contact with a syphilis sore during vaginal, anal, or oral sex. The bacterium that causes it, called a spirochete, can penetrate intact mucous membranes or tiny skin abrasions and reach the bloodstream within hours, making it highly efficient at establishing infection from a single encounter.
How the Bacteria Gets In
Syphilis is caused by a corkscrew-shaped bacterium that burrows through the moist, thin tissue lining the genitals, rectum, and mouth. It can also enter through microscopic cuts or abrasions on the skin. Once it crosses that barrier, it quickly spreads through the lymphatic system and blood, becoming a whole-body infection even though the initial sore appears in just one spot.
This is why syphilis can infect areas that might seem unexpected. A sore on the lips, tongue, or throat is enough to pass the infection during oral sex. In a study of gay men diagnosed with syphilis, 1 out of 5 reported having only oral sex as their exposure. The bacterium doesn’t need a visible wound to get in. Intact mucous membranes are enough.
Sexual Contact Is the Primary Route
The overwhelming majority of syphilis cases come from direct skin-to-skin or skin-to-mucous-membrane contact with an active sore. These sores, called chancres, typically appear at the site where the bacterium entered the body: the genitals, anus, rectum, or mouth. They’re usually firm, round, and painless, which means many people don’t notice them at all.
Condoms reduce the risk by preventing contact with a sore, but they don’t eliminate it. Sores sometimes appear in areas a condom doesn’t cover, like the scrotum, inner thigh, or around the mouth. Contact with those exposed sores can still transmit the infection. During the secondary stage, when a widespread rash and moist patches can develop on mucous membranes, the infection remains transmissible through the same kind of direct contact.
Ways You Cannot Get Syphilis
The bacterium dies quickly outside the human body. It cannot survive drying out or normal exposure to oxygen, and it only lasts a few hours even in stored blood or plasma at room temperature. This means toilet seats, doorknobs, shared towels, swimming pools, and hot tubs are not realistic sources of infection. You also can’t get syphilis from sharing utensils or drinking glasses.
Blood transfusion was once a documented route of transmission, but no transfusion-related case has been reported in the United States in more than 40 years. Modern blood screening has essentially eliminated that risk. Sharing needles carries a theoretical risk because of blood-to-blood contact, but sexual transmission accounts for nearly all cases.
Passing Syphilis During Pregnancy
A pregnant person with syphilis can pass the infection to their baby during pregnancy or delivery. This is called congenital syphilis, and it can cause serious complications including stillbirth, bone abnormalities, and organ damage. The U.S. Preventive Services Task Force gives its highest recommendation (Grade A) for universal syphilis screening as early as possible in pregnancy. Major medical organizations also recommend rescreening in the third trimester and again at delivery for those at higher risk.
When Someone Is Most Contagious
Syphilis is most contagious during its first two stages. In the primary stage, the painless sore that appears at the infection site is teeming with bacteria. Because these sores don’t hurt, people often don’t know they have one, especially if it’s inside the rectum, vagina, or mouth. The sore heals on its own within a few weeks, but that doesn’t mean the infection is gone.
The secondary stage follows weeks to months later and can include a rash on the palms, soles of the feet, or other body areas, along with moist, flat patches in the mouth or genital region. These lesions are also infectious. After the secondary stage resolves, the infection enters a latent phase where there are no visible symptoms and the person is generally not contagious through sexual contact, though vertical transmission to a fetus remains possible.
How Common It Is Now
Syphilis rates have been climbing sharply. Provisional 2024 data from the CDC shows a rate of about 56 cases per 100,000 people in the United States across all stages, including congenital cases. That represents a significant increase compared to historic lows seen in the early 2000s. The rise affects multiple demographics, but men who have sex with men continue to carry a disproportionate share of cases.
Because syphilis sores are painless and sometimes hidden, many people spread the infection without knowing they have it. Testing is the only reliable way to know your status. A simple blood test can detect syphilis, and the infection is curable with appropriate antibiotic treatment, especially when caught early.