Definition:

A natural phenomenon in which microbes acquire genes that enable resistance to antimicrobial agents. Patients can develop an AMR infection in one of two ways: 1. They can contract a resistant microbe infected by an agent that is resistant to medicines from the start. 2. A resistant microbe can emerge in their system after they start antimicrobial treatment. Resistant infections arising in this way, result from a treatment that killed off the susceptible microbes, but left resistant ones behind to flourish.

Source: http://emerald.tufts.edu/med/apua/about_issue/about_antibioticres.shtml