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From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

MELISSA BLOCK, HOST:

And I'm Melissa Block. Today in the journal Science, researchers announced the discovery of the largest virus ever seen and they gave it a name that sounds like something out of a sci-fi thriller. So should we be worried? To answer that question, here's NPR's Geoff Brumfiel.

GEOFF BRUMFIEL, BYLINE: There are a lot of scary viruses running around these days, so when scientists discover a big virus and call it Pandoravirus, you've just got to ask. This is not some giant virus that will go rampaging through the cities, knocking over buildings?

EUGENE KOONIN: No, no, no. We can be completely, completely relaxed about such prospects. They do not exist.

BRUMFIEL: Eugene Koonin is a researcher at the National Institutes of Health who studies viruses.

KOONIN: This is not going to cause any kind of widespread and acute illness or epidemic or anything, never ever.

BRUMFIEL: But Koonin is still interested in the new virus because it's freakishly bit. Is it as big as a housefly?

KOONIN: No. But you can definitely see it very well with a regular microscope.

BRUMFIEL: And that's huge in the virus world. Remember, a normal virus is tiny. It's just a sack of genetic material that injects itself into a cell. It can't reproduce on its own, so it hijacks the cell's machinery to churn out more viruses. The Pandoravirus is so big, it's hard to tell it's even a virus, says Jean-Michel Claverie, the researcher at Aix-Marseille University in France who discovered it.

JEAN-MICHEL CLAVERIE: They don't have a regular shape, like regular viruses. They really look like blobs.

BRUMFIEL: Claverie first stumbled across giant viruses a decade ago, when another researcher brought him one that was misidentified as bacteria. This time, he and his team went on the hunt for big viruses. They'd seen hints that they might live underwater, so they teamed up with oceanographers and scooped out sediment samples from the coast of Chile and a freshwater pond in Australia.

CLAVERIE: We bring back the sample and we put the sample in contact with our laboratory amoebas.

BRUMFIEL: Amoebas are microscopic critters and the team used them to tell if there are giant viruses in a sample. Amoebas eat the viruses and get sick.

CHANTAL ABERGEL: If they die, we suspect that there's something in there that killed them.

BRUMFIEL: Chantal Abergel is Claverie's co-author. She's also his wife.

ABERGEL: And it worked.

BRUMFIEL: The amoebas died and the team found that they had two kinds of new Pandoravirus. When they sequenced the genomes of the new viruses, they got a shock. The genetic code is about twice as big as any virus found to date and it seems almost completely unlike anything else on the planet. Claverie says he thinks the Pandoravirus may have started as an ancient form of life.

CLAVERIE: We believe that those new Pandoraviruses have emerged from an ancestral cellular type that no longer exists.

BRUMFIEL: Eugene Koonin, who wasn't involved with the research, isn't buying this theory.

KOONIN: These viruses, unusual as they might be, are still related to other much smaller viruses.

BRUMFIEL: Koonin thinks that the Pandoravirus' bulky genome may be a mishmash of random genetic material it sucks up from the things it's infected. He says the hunt for more giant viruses is only just beginning.

KOONIN: We are going to see many, many more giant viruses discovered around the world, some of which probably will be bigger than Pandoraviruses.

BRUMFIEL: Meanwhile, Claverie is looking at what Pandoravirus actually does in the wild. The fact that it can be found in Australia and Chile, in fresh water and salt, suggests it may be a big player in underwater ecosystems around the globe. Geoff Brumfiel, NPR News.

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