A sexually transmitted infection (STI) that almost disappeared is back — with a vengeance.

The medieval STI can silently but significantly increase the risk of heart attack, stroke and dangerous swelling of the aorta, the body’s largest blood vessel, a large new study found.

Once nearly eradicated in the 1990s thanks to safer sexual practices, cases of syphilis are on the rise due to decreased condom use, less screening and pandemic-related disruptions in healthcare access.

The bacterium Treponema pallidum causes syphilis, which can spread through contact with infectious sores during vaginal, anal or oral sex.

The infection progresses through four stages — primary, secondary, latent and tertiary — if it’s not treated with antibiotics.

The researchers of the new study looked at health records of 1,469 adults diagnosed with syphilis between 2011 and 2025 and patients without the STI, but with similar health profiles.

In what’s believed to be the first modern study to identify an independent link between syphilis and heart and blood vessel health, researchers found that syphilis bacteria damage blood vessels over time without obvious signs like skin rash, fever or fatigue.

Those with syphilis developed cardiovascular issues at a much higher rate, with heart attacks occurring in nearly 7% of patients and strokes affecting more than 10%.

Plus, the risk of a bleeding stroke (when a blood vessel in the brain breaks and bleeds) was 92% higher, while strokes caused by a blocked blood vessel were 53% more likely.

Those infected also had roughly twice the risk of aortic swelling and had a 28% higher risk of reduced blood flow to the limbs, which has similar long-term dangers as coronary artery disease.

Even those who had the STI but showed no symptoms had higher risks of death, aortic problems and stroke.

The most telling results from the study were that late-stage syphilis patients had the most severe risks compared to non-infected patients.

The risk of death was nearly six times higher, and aortic swelling was more than five times higher.

Risk of stroke tripled, while heart attacks had more than a twofold increase, and even heart failure and irregular heart rhythms were twice as common.

However, patients with early-stage syphilis showed no statistically significant increases in heart or blood vessel issues.

While the findings, published in JAMA Network Open, don’t show that syphilis is a direct cause of these heart issues, the researchers say the data is hard to ignore.

In addition to STI testing for the general public and those with unexplained blood vessel disease, researchers also recommend routine heart health screening for those diagnosed with syphilis, as early diagnosis could be life-saving.