The MRSA 'superbug' is a bacterium that has evolved in hospitals though natural selection. It is resisant to commonly used antiboitics.
Some pathogens, particularly viruses, can mutate resulting in new forms.
Very few people are immune to these new pathogens and so disease can spread quickly within a country (epidemic) or across countries (pandemic.) These are very difficult to control.
REMEMBER: Mutation happens instantly and natural selection takes many years.
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