Here’s how it works. Roughly 70% of all antibiotics administered in the US go to animals in the food supply. Some are used to treat active illnesses. Many are administered for “nonthereputic purposes.” For those of us who slept through Pharmacology 101, that means they were given to healthy livestock to make them grow faster and to prevent illnesses.
There’s mounting evidence that low levels of antibiotics given over long periods of time increase the presence of drug resistant bacteria in cows, chicken, and pigs. When humans come in contact with this livestock---either on a farm or on their fork---the drug resistant microbes or the antibiotics can be transferred. A recent CDC study showed that 20% of deaths due to drug resistant staph infections in the Netherlands could be traced back to animals.