Since the beginning of medicine, we've attacked the organisms that make us sick. We've swallowed antibiotics to fight bacteria. We've poisoned parasites. We've vaccinated and boosted our immune systems to quell infections. We've put a lot of pressure on the pathogens. The only ones that can survive are those that resist our treatment.
Resistant organisms often survive because they can breed and share genes quickly, faster than we can kill them. Even if we stamp out an infection in a single person, it is difficult to drive a particular pathogen extinct because often some survive in someone, somewhere in the world. There are many reasons for that: People can't afford the effective drugs, they take the wrong drugs or the wrong dose, or they don't take them for the right amount of time. Overall, our efforts to treat infectious disease are driving microbes toward resistance, which means we need new treatments. This arms race will continue as long as we try to suppress or kill organisms that invade us
Life & Earth Science, "10 Predictions for Evolution Over the Next Million Years", [online], Available: http://science.discovery.com/life-earth-science/10-predictions-for-evolution.htm