Today, many bacteria that most frequently cause hospital-acquired infections are resistant to the preferred antibiotic for treatment. Microbes are challenging us in ways we wouldn't have imagined 10 years ago and for which we're not well prepared. On a good day, we hold them at bay. On a bad day they're winning.
Natural selection plays a big role.
Most bacteria which are exposed to an antibiotic are killed. This situation is made worse when patients fail to follow doctors' instructions for the use of antibiotics or when physicians prescribe antibiotics inappropriately. At the same time, world travel and the globalization of our food supply are exposing us to an increasingly wide variety of microbes. Also increasing our risk is poor hygiene - the way we wash our hands and handle foods. There are scores of known bacterial diseases that no longer respond to older antibiotics such as penicillin.