Why does evolution occur more faster in bacteria while developing resistance to antibiotics?

Bacteria undergo genetic mutations at a much quicker rate than humans. Often this will prove fatal to the mutated bacteria, but since bacteria are so plentiful, and since they reproduce so quickly, this poses no threat to the overall bacteria populations.

Every once in a while, just by chance, one of these genetic mutations causes the bacteria to become resistant to a particular antibiotic. For example, a bacteria's DNA may mutate and begin producing a chemical which inactivates penicillin.

Now imagine that the mutated bacteria finds its way into a human host. When the human realizes she is sick and begins taking penicillin, all of the non-mutated bacteria cells are killed, leaving only the mutant bacteria to thrive. It can reproduce without risk from the penicillin, and in short order the patient has an infection consisting ENTIRELY of mutated, penicillin resistant bacteria. The patient can then spread the infection to others, and penicillin will prove useless to combat the illness.

In short, it is the frequent genetic mutation, large bacteria population, and short reproductive time which allows such quick resistance to develop.