Methicillin-resistant Staphylococcus aureus is the bacterium responsible for several difficult-to-treat infections. Frome Wiki
MRSA is any strain of Staphylococcus aureus that has developed, through the process of natural selection, resistance to beta-lactam antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins.
MRSA infections are more difficult to treat and thus more dangerous.
MRSA is especially troublesome in hospitals, prisons and nursing homes. Places where patients with open wounds, invasive devices, and weakened immune systems are at greater risk of infection than the general public.