Speckled with lesions across its dried, ghastly face, the nearly 500-year-old child mummy buried in a church in Naples, Italy, was long thought to be the earliest evidence of smallpox during the Italian Renaissance period.

But decades after researchers first examined it, a group of scientists has taken another look at the toddler’s remains and discovered that it most likely suffered from a different scourge: the hepaтιтis B virus.

The finding, published Thursday in the journal PLOS Pathogens, provides further evidence that the hepaтιтis B virus has afflicted humans for centuries. Today more than 250 million people live with the virus worldwide and it killed nearly 900,000 people in 2015, mostly through liver disease, according to the World Health Organization.

“This very common and nasty bug has been with us at least 500 years, and I suspect it may have been here many, many thousands of years,” said Edward Holmes, an evolutionary biologist from the University of Sydney and an author on the paper.

The mummified child, who died at two years old, was buried in the Basilica of Saint Domenico Maggiore in Naples during the 16th century. The researchers who have worked there acknowledge some of the emotional challenges to studying the toddler’s remains.

“There’s this hollowness yet this ghostlike pain still there which is fascinating from a scientific perspective but horrific from a parental perspective,” said Hendrik Poinar, an evolutionary geneticist at McMaster University in Ontario and an author of the recent paper.

In the crypt, the child’s coffin rests near dozens of other wooden burial boxes, some of which held the bodies of Aragonese princes and Neapolitan nobles. Their corpses were clothed in decorated woven fabrics and precious silk. Many were embalmed but some were naturally mummified by the dry conditions of the basilica.

“The mummies of San Domenico Maggiore are unique in Italy not only for the antiquity and excellent state of preservation of the bodies,” Gino Fornaciari, a paleopathologist at the University of Pisa and an author of the paper, said in an email, “but also for the fame of the personages, whose lives and causes of death are well known.”

Dr. Fornaciari first studied the child mummy in the 1980s. Using an electron microscope he found what he thought were traces of smallpox, also known as variola virus, in its remains. Since then the child has served as a quintessential example of early European smallpox. That made it a prime target for Dr. Holmes and his colleagues who have tried to map out the timeline and diversity of the smallpox virus.

In 2016, Dr. Holmes and his team had found traces of smallpox in a 17th century Lithuanian mummy. By re-examining the Naples mummy with molecular tools, they expected to push back the timeline of smallpox in Europe another hundred years.

“That was the hope, that this would be a slam dunk,” Dr. Poinar said. “But it’s never a slam dunk when you think it’s a slam dunk.”tamination,” Dr. Poinar said.