"Antibiotics are modern wonder drugs, but their weakness is that they can gradually put themselves out of business. Use them too much, and some of the bugs they routinely control could mutate into resistant variants that require stronger or newer antibiotics to overcome.
This is an increasingly frightening problem. Estimates are that more than 90,000 hospital patients die every year from drug-resistant bacteria, and still more people die from 'superbugs' they pick up outside hospitals. That's why doctors discourage patients from turning reflexively to antibiotics for every minor sniffle. Overuse can encourage the evolution of mutations that shrug off routine drugs such as penicillin or tetracycline and require exotic new antibiotics — or in some cases can't be killed at all.
But at least humans usually have to be sick and get a prescription from a doctor to obtain an antibiotic. Not so with pigs, chicken, cattle and other 'food animals,' which routinely get the drugs to make them grow faster and bigger and ward off diseases they might get from being crowded together in modern factory farms."