Question: why are red squirrels bad?
Answer: Unfortunately the grey squirrel carries the squirrel Parapox virus. It's a bit like Myxomatosis for squirrels and whilst it doesn't seem to harm the greys, it kills the reds. Between 1900 and 1920 (just a few decades after the arrival of the greys in this country), the reds were almost wiped out by the virus.