Syphilis is a bacterial infection, not a viral one. It is caused by a corkscrew-shaped bacterium called Treponema pallidum. This distinction matters because bacterial infections like syphilis can be cured with antibiotics, while viral infections generally cannot. Syphilis remains one of the most common sexually transmitted infections in the United States, with over 190,000 total cases reported in 2024.
Why Syphilis Is Bacterial, Not Viral
The confusion is understandable. Syphilis shares some surface-level similarities with viral STIs like herpes: it can cause sores, goes through periods of no symptoms, and is spread through sexual contact. But the underlying biology is completely different.
Viruses are not living cells. They hijack your own cells to reproduce and are extremely difficult to eliminate once established. Bacterial infections, by contrast, are caused by single-celled organisms that can be targeted and killed by antibiotics. Treponema pallidum is a bacterium with its own cell structure, though it is an unusual one. It has very few proteins on its outer surface, which helps it dodge your immune system, and it depends almost entirely on its human host for nutrients. Scientists were unable to grow it in a lab culture until 2018, which is part of why syphilis research has historically been difficult.
The practical takeaway: because syphilis is bacterial, it is curable. The standard treatment is penicillin, which has remained effective against the bacterium for decades. People who are allergic to penicillin can be given alternative antibiotics, though pregnant patients with a penicillin allergy are typically desensitized and treated with penicillin anyway, because it is the most reliable option for protecting the baby.
How Syphilis Spreads
Syphilis spreads primarily through direct contact with an infectious sore during sexual activity. These sores, called chancres, can appear on or around the genitals, anus, rectum, lips, or mouth. The bacterium enters through contact with these open lesions.
Syphilis can also pass from a pregnant person to their baby through the placenta. This is called congenital syphilis, and the risk of transmission is high, between 60% and 80%. In 2024, nearly 4,000 cases of congenital syphilis were reported in the U.S., marking the 12th consecutive year of increases. Untreated syphilis during pregnancy can cause miscarriage, stillbirth, premature birth, and serious health problems in newborns including deformed bones, organ damage, blindness, and deafness.
The Four Stages of Infection
One reason syphilis is sometimes mistaken for a viral infection is its unusual progression through distinct stages, with long quiet periods in between. This pattern looks more like a chronic viral infection than a typical bacterial one.
Primary Stage
A firm, round, painless sore appears at the site where the bacterium entered the body. Because it doesn’t hurt, many people never notice it. The sore lasts 3 to 6 weeks and heals on its own whether or not you get treatment. Healing does not mean the infection is gone.
Secondary Stage
Weeks to months later, a rash may develop, often on the palms of the hands or soles of the feet. It tends to be rough and reddish-brown, and it usually doesn’t itch. Other symptoms at this stage can include fever, swollen lymph nodes, sore throat, patchy hair loss, headaches, weight loss, muscle aches, and fatigue. These symptoms also resolve on their own, which can create a false sense that the problem has passed.
Latent Stage
During the latent stage, there are no visible signs or symptoms at all. Without treatment, the bacterium remains in the body and this stage can last for years.
Tertiary Stage
If still untreated after 10 to 30 years, syphilis can damage internal organs, including the heart, blood vessels, brain, and nervous system. Tertiary syphilis can be fatal. At any stage, the infection can also spread to the brain (neurosyphilis), the eyes (ocular syphilis, which can cause blindness), or the ears (otosyphilis, which can cause hearing loss and vertigo).
How Syphilis Is Diagnosed
Diagnosis typically involves two types of blood tests used in sequence. The first is a screening test that looks for antibodies your immune system produces in response to infection. The most common versions are called RPR and VDRL. These tests can sometimes return false positives from autoimmune diseases or other infections, so a positive result is confirmed with a second, more specific test that detects antibodies made only in response to syphilis.
If a screening test and confirmatory test are both positive, you either have a current infection or had one in the past that was already treated. Your doctor can distinguish between the two based on your history and antibody levels.
Treatment and What to Expect
Penicillin is the gold standard treatment for all stages of syphilis. The specific formulation and number of doses depend on how long you’ve been infected. Early-stage syphilis is often treated with a single injection. Later stages require longer courses.
Within the first 24 hours after treatment, some people experience what’s known as a Jarisch-Herxheimer reaction: a temporary spike in fever, headache, and muscle aches. This is a response to the bacteria dying off, not an allergic reaction to the medication. It typically passes quickly.
Early treatment completely cures the infection and prevents progression to later stages. Treatment at later stages can stop further damage but cannot reverse organ damage that has already occurred. Primary and secondary syphilis cases declined 22% from 2023 to 2024 in the U.S., a sign that public health screening efforts are having an impact, but tens of thousands of new cases still occur each year.