The reason is that your body is host to hundreds of different species of bacteria, fungi and other microorganisms. Every part of your body that is exposed to the outside world--eyes, ears, skin digestive tract, nose, mouth and throat--has its own variety of microorganisms. You don't get sick from them because:However, this is controversial and not accepted by many doctors;
1. they destroy each other and
2. your immune system controls the numbers
Antibiotics destroy some microorganisms and not others, changing this balance throughout your body; and no one knows whether that's good or bad. But it's certain that the microorganisms in your body will become resistant to the antibiotic (antibiotics kill the sensitive ones, the resistant ones are left). If you later get a serious infection and need those drugs, possibly to save your life, they may not work.