Well it's true that germs can evolve faster, partly because their generation turnover is faster, but there are complications with the way they reproduce...can o' worms there...it's quite likely to be natural selection allowing resistant strains that might have already been present to take over. Remember ye olde peppered moth from school - the black ones didn't evolve as a result of pollution, they were already there in low numbers before the big smoke, but the change in conditions suddenly gave them selective advantage. So the story goes, but who was that bloke who fiddled the results...science, eh? :-)