In the late fifties the antibiotic methicillin was introduced to combat the penicillin resistant strains of the bacteria Staphylococcus aureus. Within a couple of years methicilin resistant strains had emerged just about everywhere methicillin was being used. The methicillin resistant varieties were possibly a direct response to methicillin (my favoured theory) or a chance hybridisation with a distantly related bacteria that already contained the methicillin resistant gene. Nobody knows for sure.
