A poxviral disease important in some red squirrel declines. Differential susceptibility and field exposure both matter when attributing population change. [ref]
Parapoxvirus causes a deleterious disease in red squirrels associated with UK population declines by Daniel M. Tompkins and others. Proceedings of the Royal Society B 269, pp. 529–533; contrasting experimental susceptibility of red and grey squirrels
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