A mummified child long thought to have suffered from smallpox may have had an entirely different affliction, according to a new study from PLOS Pathogens. A recent look at the DNA of the 16th-century child led researchers to believe their initial diagnosis was wrong, as the mummy may have had Hepatitis B instead, Gizmodo reported.

Smallpox was a highly contagious virus that would have been a serious threat in the 1500s, when the child was mummified and buried in Naples, Italy. Due to a visible rash on the child’s body, scientists in the 1980s concluded it likely suffered from smallpox. However, testing during this latest study found DNA belonging to hepatitis B on the child, potentially meaning the rash was instead caused by Gianotti Crosti syndrome.

Hepatitis B is commonly thought of as a sexuallty transmitted disease, but that is only one of the ways in which is can be passed on. It can also be transmitted from a mother to her child. It is estimated that hundreds of millions of people in the world live with hepatitis B, with the disease causing almost 900,000 deaths in 2015, according to the World Health Organization.

The most noteworthy aspect of this potential discovery is that the hepatitis B appeared almost identical to a modern version of the virus. These things typically mutate over time, but if this child did indeed have hepatitis B, it might mean the virus has remained relatively steady over the past 500 years.