Grey squirrel

The Ninth Doctor noted that grey squirrels carried a pox which was deadly to their tufty "cousins", the red squirrels; the greys were immune, but the reds were not. As a result, red squirrels were not likely to be seen in England in the 2000s; indeed, Rose Tyler, had never seen a red squirrel when the Doctor compared the pox to the Raldoma Syndrome carried by Horlak's tribe of Silurians. (COMIC: The Lost Dimension ^[+]Loading...["The Lost Dimension (comic story)"])