Grey squirrels do carry a disease but they are not the main reason why red squirrels are endangered. It’s more that red squirrels are losing their habitats, due to industry chopping down conifer forests (hence why they still thrive in Northumberland and the Scottish highlands). Red forests can’t digest acorns, so losing their habitat means losing their food, which helps to build immunity against poxvirus.
Despite wildlife experts calling for an end to culling grey squirrels, with charity Wildlife Aid saying not allowing grey squirrels to be rescued just means they go underground, due to ill-informed decisions by government, not based on science of those who know how to help both species.
The answer to restoring red squirrel population is to plant pine trees, to naturally help them build up immunity by providing homes and foods they can digest. Grey squirrels tend to live more on the ground (nature then takes care of itself through natural predators like pine martens). Oral contraception to grey squirrels could also help.
Feeding any squirrels artificially is not good, as greys then depend on humans, and red squirrels get ill as they can’t digest acorns (don’t use squirrel-proof bird-feeders, as clever squirresl get inside, then get trapped). Also avoid using items made from squirrel fur (make-up brushes, artist brushes, fur trim).
For more information, Professor Acorn is a very informative website (authored by a grey squirrel!) who goes into detail on the science behind why grey squirrels are unfairly targeted, and how to protect endangered red squirrels, without harming either species. Two good welfare sites for information and help are Squirrels Info and Urban Squirrels.
Read A Scurry of Squirrels. This book is by Polly Pullar, who helped the population of red squirrels not by culling greys, but by planting over 5000 trees and dense hedgerows on her farm in the Highlands, leaving areas untouched to ‘let nature manage nature’. She has also reared numerous litters of orphaned kittens, to eventually returning them to the wild. 70% of red squirrels now live north of the border, proving that her methods work. Also read The True History of Grey Squirrels in Britain for a good overview of the issues, and humane solutions by John Bryant, who was England’s top consultant on humane wildlife control, until his death.