Conservationists are warning that a virus is posing a threat to squirrels living in Northumberland and Cumbria.
Most of England's red squirrel population is in the North
Cases of squirrel pox have been found in southern Scotland and 20 miles south of Kielder Forest, in Northumberland.
It is carried by grey squirrels, which are relatively unaffected by it and massively outnumber the red variety.
Infected red squirrels become lethargic, suffer lesions and tend to die in about two weeks from the virus or from secondary infections.
The virus is in the same group as myxomatosis and smallpox.
Disease flares up
Louise Bessant, from Northumberland Wildlife Trust, said they were not sure how the virus is transmitted and there was no cure.
She said it appeared the grey squirrels had antibodies to protect themselves.
She said: "It has been spreading up the country over the years and what happens is the reds die out, the disease flares up and burns out and then the greys can step into their place much quicker than if it was only food competition that was involved."
She said research suggested if the virus did reach Kielder Forest, it could result in hundreds of red squirrel deaths, but would not cause compete extinction.
People can help to stop the spread of the virus by keeping bird and squirrel feeders clean and not putting out food where they see red and grey squirrels together.
They can also contact wildlife groups when they see sick or dead squirrels, so more research can be done into the virus.