Multi-drug resistant strains of bacteria, such as MRSA or VRSA, can arise from resistance arising to a different antibiotic in a strain that was resistant to a different antibiotic. Where efflux pumps have been developed by the bacteria, it may be possible for the bacteria to pump out other antibiotics and not just the original one it was developed in response to.
Isolation of infected patients can be used to reduce the spread to other patients in a clinical setting, as well as putting more stringent disinfection protocols into place.