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Now, evidence of a worrying new flu virus in China. The World Health Organization says 16 people in a wide area in and around Shanghai have come down with the flu. Six have died. This new virus has touched off a major effort among scientists to determine what kind of threat it might present. NPR's Richard Knox explains.

RICHARD KNOX, BYLINE: To understand why so few cases are generating so much concern, the first thing you need to know is that no flu virus like this one, called H7N9, has ever been known to infect humans before.

THOMAS FRIEDEN: Since it doesn't affect people, people haven't developed immunity to it.

KNOX: That's Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention. The lack of immunity among humans is one key requirement for a pandemic. The other is a germ's contagiousness.

FRIEDEN: Does it spread readily from one person to another? And everything we're seeing at this point suggests that is not happening.

KNOX: This past week, Chinese authorities have intensively investigated 100 close contacts of the 16 flu cases.

FRIEDEN: And none of them have developed illness. In contrast, in influenza, you would expect at least 20 to 30 percent of family members to develop illness. So the fact that there are that many contacts and no illness has been identified is encouraging.

KNOX: But flu experts would be a lot more comfortable if they knew where this unique strain of flu is coming from and how it's been getting into humans. Gregory Hartl is a spokesman for the World Health Organization.

GREGORY HARTL: The most important thing is to try to understand how this virus gets around. We need to know the behavior of this virus in order to know how to control it.

KNOX: Research so far tells them this much: It's a bird virus, but it doesn't kill birds or even make them very sick. So it's hard to track through infected flocks. It's been evolving under the radar for quite some time, picking up mutations that allow it to infect mammalian cells, including ours. The best bet is that people are catching it from poultry so the Chinese are beginning to destroy thousands of birds.

Nancy Cox, the CDC's top flu virologist, says she's trying not to waste time merely worrying.

NANCY COX: All of our energy is going into being prepared for whatever might come.

KNOX: The virus might become a public health threat or it might fizzle. Either is possible at this point, she says. Richard Knox, NPR News.

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