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This is WEEKEND EDITION, from NPR News. I'm Rachel Martin. Public health experts have been anxiously watching China over the past couple weeks. A new flu virus there has infected dozens of people over a wide area around Shanghai. Most recently, a 7-year-old in Beijing contracted the virus. NPR's Richard Knox says flu researchers are trying to figure out what sort of threat this virus poses.

RICHARD KNOX, BYLINE: Last Thursday, a precious package arrived at the Centers for Disease Control and Prevention in Atlanta. Inside, packed in dry ice to keep it frozen, was a vial of viruses taken from a 35-year-old Chinese housewife who died last Tuesday. The package was addressed to the CDC's top flu virologist, Nancy Cox.

NANCY COX: Once we got the virus, we took it immediately to the appropriate level of biocontainment.

KNOX: That's a so-called Biosafety-3 lab, where researchers can keep this dangerous virus under tight control.

COX: We unpacked it from the various levels of protection - that is, containers in which it is placed in order to ensure that it doesn't spill. And then the work actually began.

KNOX: There's a lot of urgent work to do, according to scientists in far-flung labs who also got samples of the virus at the end of last week. In London, virologist John McCauley wants to develop a reliable test to see who's been infected with H7N9, as this virus is called.

JOHN MCCAULEY: The top priority is diagnosis - capability of the global network to be able to pick up this virus should it emerge outside of China.

KNOX: At St. Jude's Research Hospital in Memphis, flu researcher Richard Webby is getting started on a vaccine.

RICHARD WEBBY: We'll start preparing some vaccine seed strains against it, you know, just in case.

KNOX: Having a seed strain is the first step in making a vaccine. CDC scientists have a long list of projects. Cox plans to infect ferrets, weasel-like mammals considered close to humans in the way they respond to flu viruses. She especially wants to see if ferrets can pass this virus on to other ferrets.

COX: We are very eager to do the transmission studies in ferrets. We're very keen to develop antiserum to this virus.

KNOX: Having an antiserum will enable scientists to tell if people close to the known cases in China have been infected without getting seriously ill. There are many mysteries to solve. One of the most puzzling things is how H7N9 has infected people in such a broad area of southeast China. Poultry and wild birds are currently the only known source of infection. But last week the Chinese reported only 14 infected birds in five different markets. Epidemiologist Marc-Alain Widdowson is in charge of tracking the infections at the CDC.

MARC-ALAIN WIDDOWSON: It does look as if the infected poultry have dispersed throughout this large area of roughly 400 miles by 400 miles. And it's likely, I think, that there will be more cases in adjacent provinces in the weeks to come.

KNOX: Cases of H7N9 have been increasing at a faster pace than cases of another deadly bird flu virus called H5N1, which researchers have been watching for more than 10 years. But Widdowson says cases of this new virus may soon go up even more because the Chinese have begun testing people for it nationally. So far, he's reassured that monitoring of nearly 800 close contacts of H7N9 cases hasn't turned up firm evidence that this virus can be spread easily. But that doesn't mean it won't happen. Nancy Cox notes the virus already has two genetic changes that enable it to infect human cells and reproduce at humans' body temperature.

COX: Which means that these viruses may already be partially adapted - not totally adapted but partially adapted - towards the type of virus that might efficiently transmit from human to human.

KNOX: She says the coming weeks may tell whether H7N9 is only a bird disease that occasionally spills over into humans, the potential seed of a dangerous global flu pandemic or something in between.

Richard Knox, NPR News.

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