Red squirrels at a National Trust reserve in Merseyside have shown signs of resistance to the pox virus that has blighted the species, say researchers.
Scientists from the University of Liverpool have studied the squirrels at the Formby site for four years.
They found that 10% of its squirrels had pox antibodies in their blood.
These antibodies are chemical tags that allow the body to recognise and respond to an infection quickly.
This strongly suggests that the squirrels have encountered the pox virus previously and recovered from it.
"Before we started this project, it was debatable whether any squirrels had survived exposure to the virus," said Tim Dale, the project's leading researcher.
"But the work that we've done has shown that a small percentage have been exposed to the virus and they're still running around healthy in the forest."
The UK population of red squirrels has been in decline since grey squirrels were introduced from North America by the Victorians.
As well as displacing red squirrels from their habitat, grey squirrels also carry the squirrel pox virus, which they have spread to the reds.
While most of the remaining UK red squirrels inhabit the coniferous forests of Scotland, Formby is one of a handful of protected reserves in England where grey squirrel numbers are controlled to protect the reds.
Despite this protection, a pox outbreak in 2008 devastated the red squirrel population, reducing it by 80%.
"Our research has shown that [the pox virus] was introduced by the greys. It just spread through the population and caused a lot of red squirrel deaths," said Mr Dale.
"So we wanted to find out if the remaining red squirrels had survived pox or had just been lucky enough not to be exposed."
The team started a project to systematically catch and examine individual squirrels.
As well as monitoring the population's gradual recovery, the study gathered blood samples that revealed clues that the animals had survived the disease.
This is evidence that the squirrels could be starting to develop resistance to the pox virus, which they could potentially pass on to future generations.
Signs of immunity to the lethal pox virus have been seen before - but only in dead specimens.
"It's obviously great news, but there's still a lot of work to be done," said Mr Dale. "It's a very small percentage [that have the antibodies] and whether that's enough to pass it on to the next generation we just don't know.
The National Trust manager at Formby, Andrew Brockbank, said the work at such protected sites was very important for the future of the species.
"This research will bring value not only to Formby, but to other sites facing similar challenges," he told BBC News, "and the decline of the red squirrel has been a major conservation challenge."
The scientists are now applying for funding to continue their work, and to investigate the route of transmission of the virus to work out how to combat it.