A new strain of flu that has the potential to become a pandemic has been identified in China by scientists.
It emerged recently and is carried by pigs, but can infect humans, they say.
The researchers are concerned that it could mutate further so that it can spread easily from person to person, and trigger a global outbreak.
While it is not an immediate problem, they say, it has "all the hallmarks" of being highly adapted to infect humans and needs close monitoring.
As it's new, people could have little or no immunity to the virus.
The virus, which the researchers call G4 EA H1N1, can grow and multiply in the cells that line the human airways.
They found evidence of recent infection starting in people who worked in abattoirs and the swine industry in China.
Current flu vaccines do not appear to protect against it, although they could be adapted to do so if needed.