ARI SHAPIRO, HOST:
We're going to look now at some of the mysteries behind one of the deadliest diseases humanity has ever faced. There's a lot about smallpox that remains unknown, like where it came from and how it became such a killer. Now scientists have found a clue in a 17th century mummy. NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE, BYLINE: In Lithuania, there's a church with a crypt that contains the remains of hundreds of people. A couple dozen of the bodies somehow dried out and became well-preserved natural mummies, including part of a corpse that was found there with no coffin.
ANNA DUGGAN: It's just the lower extremities, so there's no associated torso or head.
GREENFIELDBOYCE: Anna Duggin is a researcher at McMaster University in Canada. She says these legs belonged to a young child...
DUGGAN: ...Estimated to be two to four years of age...
GREENFIELDBOYCE: ...Probably a boy who lived around 1665. Her team got a bit of the mummy's skin and did DNA tests to see if they could recover any ancient viruses.
DUGGAN: Because while ancient pathogen research is kind of a booming field right now, most of what's been done is actually bacterial. There's very little viral work.
GREENFIELDBOYCE: Well, they turned up one ancient virus - smallpox. This is the oldest complete set of smallpox genes ever found. Scientists were able to compare this DNA to the DNA of more modern smallpox viruses, ones collected in a few decades before 1977. That's when a global vaccination campaign wiped out smallpox from nature. In the journal Current Biology, the researchers report that all of these strains - the ancient and more modern - seem to have shared a pretty recent common ancestor, a virus that was out there around 1588 to 1645. And biologist Paul Keim says that seems to kill some theories about smallpox.
PAUL KEIM: You know, there were other models out there that were arguing that contemporary strains had gone back thousands of years, and that doesn't look like the case.
GREENFIELDBOYCE: Keim is from Northern Arizona University. He didn't work on this study, but he has used DNA of ancient plague bacteria to trace the history of that disease. He says with only one sample of ancient smallpox DNA, you can't draw too many conclusions.
KEIM: All we can really say is that the smallpox that was trafficking around the world there at the end of its existence can be traced back to the 17th century.
GREENFIELDBOYCE: Still, the link to the early 17th century is tantalizing. Hendrik Poinar is director of McMaster University's ancient DNA center.
HENDRIK POINAR: That date is coincident with many outbreaks within Europe.
GREENFIELDBOYCE: Poinar says historical records suggest that a shift occurred about that time, with smallpox becoming more virulent, but there is evidence that some versions of the virus were around long before that. There's even signs of pockmarks on 3,000-year-old mummies from Egypt.
POINAR: Whether or not those are real smallpox or measles or something else remains, I think, a question.
GREENFIELDBOYCE: He's hoping to get more ancient DNA samples to find out. The biggest question is where smallpox came from in the first place. What's unusual about this virus is that it only infects people.
POINAR: So it seems really to be human-specific.
GREENFIELDBOYCE: Scientists think smallpox may have jumped into humans from an animal, but they don't know what animal or when or how.
POINAR: I find that fascinating. I mean, this is a major pathogen that's caused tremendous mortality, morbidity in humans.
GREENFIELDBOYCE: He says understanding its history is really important because there's other viruses out there that could pose a threat. Nell Greenfieldboyce, NPR News.
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