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Threatwatch is your early warning system for global dangers, from nuclear peril to deadly viral outbreaks. Debora MacKenzie highlights the threats to civilisation and suggests solutions

Exactly 10 years after H5N1 bird flu exploded across south-east Asia, the virus is still widespread, and has been joined by new killer types of bird flu. Human cases of H7N9 flu are surging in south-east China, and a new type of bird flu, H10N8, has claimed its second human victim, in the same region.

Now it seems that all of these viruses stem from a single, mother virus. Targeting it might stop it from spawning new, deadly viruses in the future.

Few people have heard of H9N2, but this virus was crucial in giving rise to the three dangerous bird flu viruses that have emerged so far in China H5N1, H7N9 and H10N8.

None of these viruses has yet evolved the ability to spread readily in people and potentially trigger a pandemic although we know H5N1 can, and H7N9 and H10N8 seem similar. But even if those viruses never go rogue, their cousins might, because the real problem is their common ancestor, which endowed them with the genes that make them dangerous.

"H9N2 is the enabler, the one to worry about," Robert Webster of St. Jude Children's Research Hospital in Memphis, Tennessee, told New Scientist. Bird flu is usually a gut infection in ducks, but H9N2 has evolved into a benign respiratory virus in chickens that has spread across Eurasia. When multiple flu viruses infect the same host, they can swap genes. They may be named for their various H and N surface proteins, but H5N1, H7N9 and H10N8 all got some or all of their "internal" genes from H9N2.

Those genes for the enzymes that replicate the viral genome, for example, or a protein that confuses a host's immune system can make these viruses dangerous, says Webster. Any of them might become pandemic if they acquire the right mutations to spread in people or hybridise with a normal human flu.

Closing Asia's ubiquitous live poultry markets would be the key to controlling N9N2, says Webster, as this is where H9N2 and its spin-off viruses spread, mingle and evolve and where humans catch them.

That is just what China is trying to do. As millions celebrate Lunar New Year this week with home-slaughtered poultry, Shanghai and three other cities have shut their live markets and officials are urging people to eat pre-slaughtered, frozen birds. The continued threat from China's bird flu may depend on whether that catches on. "It's the Year of the Horse in China," says Webster. "I hope they can get the stable door closed before the horse has bolted."

H7N9 emerged in south-east China last spring, infecting 136 people, a third of whom died. In response, Chinese flu scientists called for live poultry markets to be shut last April. Some were but they were re-opened when flu cases dropped in the following months.

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Threatwatch: Mother virus of China's deadly bird flu