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The H7N9 bird flu virus is being passed around from flock to flock of chickens and ducks, is mutating, and is now a bigger threat than ever to humanity, researchers reported Wednesday.

The virus, which was first reported in people just two years ago, has settled into southeastern China, Guan Yi of the University of Hong Kong and colleagues reported. Unless drastic measures are taken to eradicate it, the virus will continue to mutate, they warn in a report published in the journal Nature.

"H7N9 viruses have spread from eastern to southern China and become persistent in chickens," they wrote.

It’s also swapping genes with other types of flu viruses, giving rise to new strains. Any one of them could start a pandemic in time, they said.

"Unless effective control measures are in place ... it is reasonable to expect the H7N9 and other viruses to persist."

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“This expansion of the genetic diversity of influenza viruses in China means that unless effective control measures are in place, such as permanent closure of live poultry markets, central slaughtering and preventing inter-regional poultry transportation during disease outbreaks, and backed by systematic surveillance, it is reasonable to expect the H7N9 and other viruses to persist and cause a substantial number of severe human infections,” they wrote.

They’ve found at least 48 different subtypes.

H7N9 avian influenza has infected 622 people since 2013 and killed 227 of them.

So far, it doesn’t seem to infect people easily, and people who are infected do not seem to spread it to others much, if at all. But influenza viruses change quickly and unpredictably and if one starts passing easily from one person to another, it could spread.

The longer a strain is circulating, the more opportunity it has to infect people and mutate. The fact that this one is mutating and exchanging genetic material is worrying, Guan and colleagues said.

H5N1 was the first strain of bird flu to really worry experts, and it still is. It’s infected 784 people in 16 countries since 2003, and killed 429 of them. It’s also still circulating, mutating and swapping genes.

All flu viruses mutate easily, which is why a fresh strain of influenza pops up every few years. The last pandemic was caused by H1N1 swine flu in 2009. It wasn't an especially deadly pandemic.

But H7N9 and H5N1 both have very high death rates, and experts fear that if either one of them caused a pandemic, tens of millions of people would die.

H5N1 and H7N9 bird flu are still more adapted to birds than to people. They don’t have the genetic mutations that make them infect people easily. But since they are so mutation prone, it could happen.