You might give it to another family member, thus allowing him or her to take a short course of antibiotic and help breed resistant bacteria. Or, under warm and humid conditions, the medicine can turn into something ineffective or even toxic. Then there's the question of whether you should have been taking the drug in the first place. Sometimes, people may take antibiotics for viral infections for which antibiotics do no good, such as the cold and flu. In doing so, they are contributing to the resistance problem.
Bacteria grow quickly in the human body.
Someone who seems to have recovered from an illness may get sick again within a week or less, thanks to rapidly replicating bacteria. They have evolved sophisticated systems to prey on you. Drug companies are already exploring ways to interfere with this defense mechanism. The problem of resistant bacteria is illustrated by what it has done to pneumococcal meningitis. The treatment you get today is considerably different from what you got a decade ago, because of the potential for resistant strains that require new or more expensive drugs.