The disease is described as a major threat to the red squirrel population
The survival of red squirrels at a reserve in Merseyside is under threat from a deadly virus carried by the more common grey squirrel.
At least 30 animals in and around the Formby Squirrel Reserve, near Southport, have died from Squirrel Pox Virus since November.
Staff and volunteers are on alert and search the woods looking for sick animals to stop the disease spreading.
Anyone who sees a sick or dead red squirrel is asked to call the reserve.
When red squirrels are infected, they become lethargic and develop scabs and lesions around their mouth and eyes, staff said.
Treatment is said to be difficult and is rarely successful.
Andrew Brockbank, the National Trust's property manager at Formby, said: "It is six months since the first reports of a red squirrel showing signs of squirrel pox virus, in a private garden close to the Formby reserve.
"Late Spring is the time when we expect to see the emergence of kittens (young squirrels) and the next few months will be telling in terms of future prospects for the local population."
Staff and volunteers search the woodlands daily for sick red squirrels. Any sick animals found are taken to a local vet for assessment and care.
The disease is described as a long-term issue and a major threat to red squirrel populations.
Squirrel charity Red Alert said it would remain vigilant.