Just as dead pigs and ducks stopped washing up in rivers in China, another public health threat crops up: Chinese media announced that a strain of bird flu never before seen in humans was found in Shanghai and surrounding provinces, infecting 24 people so far in China and killing seven of them.

For the most part, flu outbreaks like this pop up then fade within weeks. But every once in a while, they become pandemics, spreading across the world and infecting people with a deadly flu. What will happen with H7N9 is unclear, but there are a few things worth knowing about its mysterious origins, and how people in China are reacting.

Where did it come from?

Unclear. The first two victims died in late February, but the Chinese state media didn’t announce the deaths or that they had detected the new virus until Sunday, March 31st. So far, it looks like all the victims contracted the flu directly from a sick bird, not another person. That’s an important distinction because a flu that humans contract from each other can spread much faster than a flu that people contract directly from animals. (A Chinese blogger has put together a map of the cases.)

The Chinese government has traced some cases back to live pigeons that were being sold in a market in Shanghai. On April 4th, Chinese state media confirmed that it had found cases of bird flu in those pigeons, and last Thursday night the government began a mass-slaughter of poultry there.

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