Latent syphilis is a stage of syphilis where the bacteria remain in your body but cause no visible symptoms. Blood tests come back positive, yet there are no sores, rashes, or other signs of infection. This “hidden” phase can last for years or even decades, and without treatment, roughly one in three people with latent syphilis will eventually develop serious complications affecting the heart, brain, or other organs.
How Latent Syphilis Differs From Other Stages
Syphilis moves through stages. The first stage produces a painless sore, usually at the site of sexual contact. The second stage brings rashes, fever, and flu-like symptoms. Both of these stages are obvious enough that many people seek care. Latent syphilis is what happens in between: the immune system pushes the infection underground, symptoms disappear, and the bacteria quietly persist.
Latent syphilis is split into categories based on timing. Early latent syphilis means the infection was likely acquired within the past 12 months. Late latent syphilis means the infection has been present for more than a year. When the timing can’t be determined, it’s classified as latent syphilis of unknown duration. These distinctions matter because they affect how contagious you are, how you’re treated, and what monitoring you’ll need afterward.
Why the Bacteria Survive Without Causing Symptoms
The syphilis bacterium, sometimes called “the stealth pathogen,” has an unusual ability to hide from the immune system. Its outer surface contains almost no recognizable proteins, roughly 1% of what’s found on a typical bacterium. This near-blank exterior makes it largely invisible to immune cells that would otherwise attack it.
During latency, the bacteria spread to hard-to-reach places: the central nervous system, the eyes, the placenta in pregnant people. These are areas where immune surveillance is naturally lower. The organisms survive in small, scattered numbers across these sites, slowly replicating. Researchers believe that by keeping their numbers low and staying spread out, the bacteria avoid triggering the kind of immune response that would clear them. They also have a trick for long-term survival: a surface protein that constantly reshuffles its genetic sequence, so even when the immune system learns to recognize one version, new variants slip through.
Is Latent Syphilis Contagious?
Early latent syphilis is still considered infectious. The estimated risk of transmission to a sexual partner during the primary, secondary, and early latent stages ranges from 51% to 64%. Early latent syphilis carries this risk because there’s about a 25% chance of relapsing back into the secondary stage, when sores and rashes reappear and the infection spreads easily through contact.
Late latent syphilis is generally not considered sexually transmissible. The bacteria have retreated deep enough into tissues that sexual contact is unlikely to pass the infection. There is one major exception: pregnancy. A person with early latent syphilis has about a 40% chance of transmitting the infection to their baby. Even in the late latent stage, that risk is around 8%. This is why routine syphilis screening during pregnancy is standard practice.
How Latent Syphilis Is Diagnosed
Because there are no symptoms to find, latent syphilis is diagnosed entirely through blood work. Two types of blood tests are used together. The first type detects general markers of infection and is useful for screening. The second type looks for antibodies specific to the syphilis bacterium. Relying on just one type can lead to a wrong diagnosis, so both are needed to confirm the infection.
Before a latent syphilis diagnosis is made, a thorough physical exam checks for any hidden signs of active disease: the mouth, genitals, perianal area, and rectum are all examined for sores or rashes that might have gone unnoticed. If any active signs are found, the diagnosis shifts to primary or secondary syphilis instead.
Classifying the infection as early versus late latent depends on the clinical history. A diagnosis of early latent syphilis requires evidence that the infection was acquired within the past year. This might come from a recent change in blood test results, a history of symptoms consistent with earlier syphilis stages, or a sexual partner who was diagnosed with early syphilis. Without that kind of evidence, the infection is classified as late latent or unknown duration.
When the Brain Needs to Be Checked
Because syphilis bacteria can settle in the nervous system during latency, some people need additional testing to rule out neurosyphilis. This involves a spinal tap to analyze the fluid surrounding the brain and spinal cord. Not everyone with latent syphilis needs this, but it’s recommended in specific situations: if you have neurological symptoms like vision changes, hearing loss, or cognitive problems; if you have untreated late latent syphilis; if blood test levels remain high after treatment; or if you’re living with HIV, which increases the risk of the infection reaching the brain.
Treatment for Early vs. Late Latent Syphilis
Penicillin remains the standard treatment, but the regimen differs depending on how long you’ve been infected. Early latent syphilis is treated with a single injection. Late latent syphilis, or latent syphilis of unknown duration, requires three injections given once a week for three consecutive weeks. The injections need to stay on schedule, with no more than two weeks between doses.
After treatment, you’ll need follow-up blood tests to confirm the infection is responding. For early latent syphilis, testing is typically done at 3, 6, and 12 months. For late latent syphilis, follow-up extends to 12 and 24 months. People living with HIV are monitored more closely, with testing at 3, 6, 12, and 24 months, and then annually. Your doctor is looking for a significant drop in antibody levels, which signals that treatment worked. If levels don’t fall as expected, further evaluation may be needed.
What Happens Without Treatment
Left untreated, about 33% of people with latent syphilis will progress to tertiary syphilis. This can take years or even decades, but the consequences are severe. Tertiary syphilis can damage the heart and major blood vessels, destroy tissue in the skin, bones, and organs, and cause irreversible neurological problems including dementia, paralysis, and blindness. The remaining two-thirds of untreated people may never develop tertiary disease, but there’s no way to predict who will and who won’t.
The scale of undiagnosed latent syphilis is significant. In the United States alone, provisional data for 2024 counted over 97,000 cases of late or unknown-duration syphilis, a rate of 28.6 per 100,000 people. Many of these cases are caught through routine screening rather than symptom-driven visits, which underscores how common it is for syphilis to go unnoticed during latency.