Lots of incorrect information about grey squirrels on this thread.
This is written by a well-regarded UK expert:
www.urbansquirrels.co.uk/have-grey-squirrels-replaced-the-native-red/
For those who choose not to read the article I'm linking to, these are a couple of key points:
Red squirrels are not a rare or endangered species, their conservation status is “least concern”. They are abundant in the rest of the world, wherever the habitat – extensive pine forests or an equivalent in wildlife corridors – is available to them.
The specifically British sub-species of the red squirrel, siurus vulgaris leucurus, is, unfortunately, extinct. This happened before grey squirrels were introduced! So the idea that once we had a lot of red squirrels, and then greys were brought in and fought them, and killed them, and drove them out, is undeniably false.
Red squirrels were then themselves reintroduced from continental Europe, so that red squirrels living in this country today are not, properly speaking, native to this country, but to Scandinavia.
In practice the vast majority of the reds do not get the pox from the greys, but from fellow reds.
In areas where the habitat is still right for red squirrels, they do coexist with the greys, they have even been known to share the dreys.