Antibiotics are an extremely powerful class of drugs that reduced mortality from many of the epidemic infectious diseases that used to be responsible for millions of deaths every year. Such diseases as syphilis and tuberculosis, which were fatal before the the discovery of penicillin, can now be cured.
Antibiotics are only useful against bacterial infections. They have no effect on viruses or fungal infections. Also, many antibiotics are effective only against specific classes of bacteria. One major issue in the developed world is over-prescription of antibiotics, often in cases where they are completely useless.
There are several problems with overuse of antibiotics. The first is economic, that people are wasting money on drugs that do not benefit them. In the individual taking the antibiotics, the antibiotics can kill off beneficial intestinal flora and breed antibiotic-resistant strains of bacteria. On a larger scale, as bacteria are numerous and reproduce quickly, over-prescription of antibiotics, routine use of antibiotics in animals, and the antibiotics introduced into the ecosystem via human waste have led to antibiotic-resistant bacteria evolving faster than we can develop new antibiotics; the more powerful antibiotics necessary to kill antibiotic-resistant strains of bacteria have more severe side effects than earlier antibiotics.