Then you go to the doctor with pneumonia or a serious diarrheal illness, and suddenly the bacteria are no longer being killed by the antibiotic that the doctor prescribes. But potentially more serious are patients in hospitals, where no antibiotic may work because some bacteria have become resistant to just about all our main antibiotics. Vancomycin-resistant enterococci , or VRE, is one of several bacteria that are being seen with increasing frequency in hospitals and that are, at best, minimally responsive to antibiotics. Keep in mind that our bodies have evolved reasonable defense mechanisms, so a healthy young adult can frequently fight off an infection without major difficulty. But when you're dealing with an older person or a young child or a person whose immune system may not be working that well, antibiotics can spell the difference between life and death.
Bacteria evolve in response to what's happening in their environment.
If there are antibiotics in the environment, most of the bacteria will be killed off. But every now and then a bacterium will acquire or develop the necessary genes to resist being killed by the antibiotics. This becomes a selective advantage, and the resistant bacteria are better able to live and multiply. So when you use antibiotics a lot, it is almost inevitable that ultimately the bacteria will figure out some way to avoid being killed. And it's taken a while for the bacteria to acquire resistance.