Syphilis is caused by a spiral-shaped bacterium called Treponema pallidum. This corkscrew-shaped organism belongs to a family of bacteria known as spirochetes, and it enters the body through mucous membranes or tiny breaks in the skin during sexual contact. Once inside, it moves fast: animal studies show spirochetes can reach the lymphatic system within 30 minutes of initial contact, making syphilis a body-wide infection almost from the start.
How the Bacterium Enters Your Body
Treponema pallidum is a remarkably specialized organism. It’s thin, coiled, and capable of burrowing through intact mucous membranes or microscopic skin abrasions. Unlike many bacteria that stay local at first, T. pallidum penetrates tissue and enters both the lymphatic system and bloodstream within hours, spreading throughout the body before symptoms ever appear.
This is why syphilis can eventually affect organs far from the original site of infection, including the brain, heart, and bones. The bacterium is fragile outside the human body, though. It can’t survive long on surfaces, which is why you cannot get syphilis from toilet seats, doorknobs, swimming pools, shared clothing, or eating utensils.
How Syphilis Spreads
The primary route of transmission is direct contact with a syphilis sore during vaginal, anal, or oral sex. These sores, called chancres, often appear on the genitals, rectum, tongue, or lips. Because chancres are typically painless and can be hidden inside the vagina or rectum, many people don’t realize they have one, which makes unknowing transmission common.
Syphilis can also pass from a pregnant person to their baby during pregnancy or delivery. This is called congenital syphilis, and it remains a serious public health concern, with nearly 4,000 cases reported in the United States in 2024 alone, marking the 12th consecutive year of increases.
What Happens After Infection
The first sign of syphilis is a small, firm, painless sore at the spot where the bacterium entered the body. Many people never notice it. The sore appears on the genitals, rectum, tongue, or lips, and because it doesn’t hurt, it’s easy to miss entirely, especially when it’s inside the body.
If untreated, syphilis progresses through distinct stages. The primary stage involves that initial sore. The secondary stage brings skin rashes and flu-like symptoms. Then comes a latent phase where symptoms disappear entirely but the bacteria remain in the body. Years or even decades later, tertiary syphilis can damage the heart, brain, nerves, bones, and other organs. At every stage before the late stage, the infection is curable with appropriate treatment.
Why It’s Hard to Detect on Your Own
Syphilis is sometimes called “the great imitator” because its symptoms overlap with many other conditions and because it can go completely silent for years during the latent phase. The painless nature of the primary chancre is a major reason infections go undiagnosed. During the secondary stage, rashes can appear on the palms of the hands and soles of the feet, but they may be mild enough to dismiss.
Diagnosis requires blood testing. Two types of tests are used together: one detects a general immune response, and the other looks for antibodies specific to T. pallidum. Both are needed to distinguish between a new, active infection and a past infection that has already been treated. Routine screening is recommended for anyone at elevated risk, including sexually active individuals with new or multiple partners.
Treatment and Curability
Penicillin remains the gold-standard treatment for syphilis at all stages. It has been the preferred antibiotic for decades, and no resistance to it has developed in T. pallidum. The specific form of penicillin and the length of treatment depend on how long the infection has been present. Early-stage syphilis typically requires a single injection, while later stages need longer courses.
Treatment can cure the infection and stop it from progressing, but it cannot reverse damage already done to organs. This is why early detection matters. After treatment, follow-up blood tests confirm the infection has cleared.
Risks During Pregnancy
When a pregnant person has untreated syphilis, the bacterium can cross the placenta and infect the developing baby. The consequences can be severe: miscarriage, stillbirth, premature birth, and low birth weight are all possible. Babies born with congenital syphilis may develop deformed bones, severe anemia, an enlarged liver or spleen, jaundice, skin rashes, and brain or nerve problems including blindness and deafness. Without treatment, congenital syphilis can be fatal. Screening during pregnancy is standard practice because treating the mother with penicillin before delivery can prevent transmission to the baby.
Current Trends in the U.S.
Primary and secondary syphilis cases in the United States declined 22% from 2023 to 2024, the second consecutive year of decline, with roughly 41,500 cases reported. Total syphilis cases across all stages reached about 190,000 in 2024. Despite the overall downward trend, congenital syphilis continues to climb, reflecting gaps in prenatal screening and treatment access. The CDC reported 3,941 congenital syphilis cases in 2024, a number that has risen every year for over a decade.