How do bacteria become resistant to antibiotics?
Antibiotics are widely viewed as having the power to completely wipe out bacteria and infections, while this is not actually true. They wipe out the vast majority if they are taken correctly and the dose is completed.
If a patient does not complete the course of antibiotics, this leaves some of the bacteria intact, and now that it has been exposed to the antibiotic, when it regains strength as an infection, it is more likely to be resistant to further treatment of the same antibiotic.
Mutation of bacteria as part of natural evolution also takes place, whereby the antibiotic kills off the majority of bacteria, but the mutated strain remains intact, and then multiplies and spreads from person to person, over time creating "superbugs", or strains that do not respond to normal courses of anti-biotic drugs.