Once, this boy mummy was thought to have died of smallpox, but a new analysis of his now 450-year-old DNA reveals signs of hepatitis B, instead — the oldest known infection of the virus. The puzzling new diagnosis is made stranger still by the similarity between the mummy’s hepatitis B virus and modern-day strains, suggesting this virus has been infecting people for thousands of years.

This tiny mummy was exhumed from an Italian church in the 1980s, and scientists have studied it ever since. Its desiccated face, arms, and torso were pitted with a rash so severe that paleopathologist Gino Fornaciari posthumously diagnosed the child with smallpox. These new findings, published in the journal PLOS Pathogens, throw that earlier diagnosis into question, though they don’t rule it out completely.

This boy died sometime during the mid-1500s, according to radiocarbon dating. It was common practice for Italian nobility at the time to be embalmed after death — and after this boy’s internal organs were removed, he was embalmed and dressed in silk clothes. The embalming process, the finery, and the body’s burial in the basilica of St. Domenico Maggiore in Naples all suggest that child came from a noble family, Fornaciari says in an email to The Verge. A co-author of the new study, Fornaciari also led the early investigations of the mummy in the 1980s that detected microscopic egg-shaped particles in the rash that looked like smallpox. Antibodies against the virus clung to these particles, supporting the ID.

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But a team of ancient DNA investigators led by Hendrik Poinar at McMaster University and Edward Holmes at the University of Sydney couldn’t find any traces of smallpox DNA from a fragment of thigh bone, muscle, or flaps of skin still sticking to the mummy’s rib, skull, and leg. Of course, the fact that smallpox wasn’t detected doesn’t mean it wasn’t there. Ancient DNA is fragile and might not have survived — although the team has detected smallpox DNA in a 17th century mummy from Lithuania. It’s possible, however, that the boy was infected by both viruses, Fornaciari says. It will just take more research to figure out.

The team turned up a different virus, though: the oldest evidence of hepatitis B, a virus that still infects more than 250 million people today. The virus spreads from person to person through bodily fluids like blood and attacks the liver — causing problems like cirrhosis and liver cancer. And it might even explain the mummy’s pockmarked skin: in children between the ages of two and six, the virus can also cause a severe rash.

Although hepatitis B is still widespread despite the safe and effective vaccine, scientists don’t know where and when it originated. The researchers hoped that reconstructing the mummy’s older version of the virus might help them understand how it had evolved over time. But when they analyzed the 450-year-old virus’s genome, they discovered that it looked surprisingly similar to a modern strain.

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This could mean that the virus came from modern contamination of the mummy, rather than a centuries-old infection (although the investigators suspect that’s unlikely). For one thing, they extracted hepatitis B DNA from all over the mummy’s body instead of just the isolated spots you’d expect if someone dribbled virus-infected fluid on it. Plus, the viral DNA was damaged in the same pattern that ancient DNA usually is.

If the mummy’s virus isn’t the result of contamination, then the findings suggest that this strain of hepatitis B hasn’t changed much over the past 450 years — which makes sense, says study author Edward Holmes. “HBV is a very unusual virus,” he told The Verge in an email. Its genome is short and rigid, so the mutations that would let it evolve could also just as easily break it. “On the one hand this makes the virus very small and efficient,” Holmes says, “but on the other it means that very few mutations actually work.”

So this remarkably efficient, slow-evolving virus must have been already well-adapted to people long before this little boy contracted it. That means hepatitis B has probably been spreading among humans for thousands, possibly tens of thousands, of years. Exactly how long, Holmes says, is still a bit of a mystery.