Every time a virus jumps from animals into humans there is a shudder of alarm.
So-called zoonotic diseases have resulted in pandemics in the past, most recently of course COVID-19.
That’s why the UK Health Security Agency is trying to work out how someone in Yorkshire has been infected with a strain of flu so similar to a virus circulating in pigs in the UK.
We don’t know much about the individual. But we do know they only had mild symptoms and fully recovered.
And they didn’t work with pigs. That’s the puzzle.
Does it mean that the virus is circulating in people, undetected until now?
NEW SWINE FLU STRAIN FOUND IN BRITISH PERSON: WHAT WE KNOW
A person in North Yorkshire has been diagnosed with a strain of flu similar to a virus circulating in pigs.
Is this swine flu? Bird flu? Both? Have there been cases in other parts of the world? What do we know about the first case?
Find out all the answers here
Health authorities are tracing contacts of the infected person and are trying to identify the source.
Flu normally crosses the species barrier by infecting people in close contact with animals.
That’s why people who work with chickens are checked for bird flu, which is caused by another strain of the virus.
But even if there is a human case of an animal virus that’s usually the end of the story. The virus isn’t passed on because it’s not well enough adapted to human cells to spread from person to person.
Officially there is no evidence of human transmission of the new swine flu virus.
But it’s hard to see an alternative explanation at the moment.
That doesn’t mean we are on the verge of a new pandemic. The strain could disappear as quickly as it emerged.
But scientists will want to understand more about the virus to gauge how transmissible it is – and also to be sure it’s not more of a risk to people who are vulnerable, either because of age or underlying health conditions.