I read that the Brown Rat eventually ousted the Black Rat from Britain and thus ended all those terrible plagues ... like the Black Death etc.. all caused by fleas carried by the Black Rat.....

The population of rats must have been massive in the Middle Ages... there were no sewers... houses made of wattle etc,
there was far more wetland in England in those days
There were thatched rooves... barns full of hay... strange to think the poor old Black Rat was eliminated comepletely .. surely a few must remain .... somewhere ?

I believe they do on a remote island off Scotland I think. Google it for details but I'm sure I saw something about it on TV.