Animals in factory farms are given doses of antibiotics -- both to keep them alive in stressful, unsanitary conditions, and to make them grow faster. The practice leads to new strains of antibiotic-resistant bacteria, such as the now-widespread form of staph (MRSA) known as ST398.
Federal regulators have in the past refused to release estimates of just how much antibiotics the livestock industry uses. But recently the FDA released its first-ever report on the topic. And the amount? Twenty-nine million pounds of antibiotics in 2009 alone.
It's important to realize that antibiotic-resistant disease like MRSA is a man-made problem, created by the excessive use of antibiotics. Medical overuse of antibiotics is one aspect, but the greatest, and most hidden, factor is the excessive use of antibiotics in food production.
Chickens, cattle and hogs are fed antibiotics, not to treat disease, but to make them grow faster, which increases profit margins for livestock producers.