One point not mentioned in the above writeups

Antibiotics generally kill off the weak bacteria first. The tougher ones are more likely to die off with extra pummelling. That is why there is usually a ten day course to be followed. When the course is stopped early, the few and the strong multiply. ACK! It's the stronger variety that has been selected to continue on and make the rest of us miserable. What we have here is evolution on a microbial scale with an extra boost from antibiotic misuse.