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There is developing news coming out Boston this morning. Police there says one of the suspects in the Boston Marathon bombing is dead and they are searching for another. We just heard from a colleague at member station WBUR who is at a building in the Boston suburb of Watertown. He says police have guns drawn and appear to be entering a building. And we'll have much more on that throughout the morning.

On to a different story for the moment. Human cases of a new type of bird flu continue to mount in China. So far 87 people across eastern China have fallen ill from the newly discovered virus. Seventeen have died. Scientists are struggling to understand what kind of threat this represents. There's no evidence the new flu can spread easily among people, though there might have been a few cases of person-to-person infection. NPR's Richard Knox says the biggest question here is how are people catching this new infection.

RICHARD KNOX, BYLINE: Dr. Keiji Fukuda is the World Health Organization's top flu official. He arrived in China today at the head of an international team of experts.

DR. KEIJI FUKUDA: The biggest question is just what's going to happen next. We've had a lot of experience in the last, you know, decade with new animal influenza viruses. But there's also a mix of things going on that we haven't seen this combination before. So I think right now, anything is still possible.

KNOX: The experts are confronted with an urgent set of unknowns.

FUKUDA: Where is the virus? Where is it keeping itself? How are people getting exposed to it? How are they getting infected by it? Those are some of the really basic questions that we're asking.

KNOX: Worry has increased recently. The virus has been found in two children in Beijing. Cases have emerged in Henan Province south of Beijing. Both are hundreds of miles from Shanghai - where the so-called H7N9 flu virus first appeared last month - and they're far removed from other outbreaks.

FUKUDA: The first thing it tells us is that however people are getting infected, you know, whatever they're getting exposed to, it's not just in one location but it's spread out.

KNOX: So far, testing of nearly 48 thousand animals, nearly all birds, has found the virus in only 39. Dr. Juan Lubroth is chief veterinary officer of the United Nations Food and Agriculture Organization.

DR. JUAN LUBROTH: There are very few positives for H7N9. I would have expected maybe more hits in the poultry sector.

KNOX: Lubroth says Chinese investigators have cast a wide net.

LUBROTH: Bird markets, poultry slaughterhouses, poultry farms, wild bird habitats, environmental sample spots.

KNOX: They looked at pigs, but found no evidence of infection. That's good, because pigs can be infected simultaneously with bird, swine and human viruses. The mingled viruses can swap genes, giving rise to new viruses that can spread among people.

The search for the animal sources of H7N9 is really only beginning. Until last week, animal tests for it weren't very good. There's now a good test, but the Chinese haven't had time to distribute it very widely.

LUBROTH: They have more samples than they have results.

KNOX: The basic problem is that H7N9 viruses are tiny needles in a gargantuan haystack. There's no way to test every last chicken, duck or pigeon.

LUBROTH: So the surveillance has to be very targeted. And because we don't have a lot of information, that target is quite dark.

KNOX: The usual tactic in this kind of situation is called trace-back. Start with the poultry markets where people who got the virus shopped or worked and then trace the source of the poultry back to the farms they came from. But Lubroth says many poultry sellers don't keep good records. Asking them where they got their chickens often leads to dead ends.

LUBROTH: They can shrug. They say there's a middleman. And then you go to the middleman and he's not there anymore.

KNOX: While the hunt is on for the virus's origins, scientists are just beginning to study it closely. Dr. Ab Osterhaus and his group in the Netherlands got samples yesterday. They want to figure out what genetic changes it would need to enable it to spread easily among people.

When they did similar experiments with another bird flu virus, critics said it was too dangerous - a virus with pandemic potential might escape from the lab. But Osterhaus says the current situation with H7N9 flu shows exactly why these experiments are important.

DR. AB OSTERHAUS: We need to understand what makes these viruses transmissible. I think that's quite important in estimating the risk in the future of these viruses that pop out in people.

KNOX: But he says they won't do the experiments without a lot of consultation with fellow researchers around the world. Richard Knox, NPR News.

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