I'd second wot slyvanius said above, normally the greys only carry the pox virus (and thankfully only the english greys) and are rarely affected by it.
Must be pretty immune to it, reds aren't and it decimates them very quickly, heard of hundreds dieing in some areas in a very shoort space of time.
If the pox ever becomes rife among the scottish cenral belt greys there spread further north will become far easier and quickier (so far only a couple of greys have ever tested positive for it furher north)
Dunno if u find many more it might be worth contacting a squirrel group incase it is a new disease?