Antibiotics
Unless the meat and dairy products you buy at your local supermarket are labeled "antibiotic free" you can be fairly certain the animal your food came from was fed an abundance of antibiotics. In fact, more than 70% of the antibiotics used in the United States is given to farm animals, largely to fatten them and to make them grow.
Why should this concern you?
One of the problems is that bacteria become resistant to antibiotics through overexposure. It will require larger doses of an antibiotic to cure an infection caused by resistant bacteria; sometimes the antibiotic will not work at all. The routine use of antibiotics on factory farms has been linked to increased incidence of antibiotic resistance in humans. The effectiveness of drugs that are needed to treat disease is being endangered.
Health experts are calling for cutbacks in the excessive application of drugs to farm animals. Subtherapeutic use of antibiotics means using them at levels that carry production claims such as increased rate of weight gain or improved feed efficiency in clinically healthy animals, or at levels generally below the optimum therapeutic level used for clinically sick animals. In the debate over the use of antibiotics in agriculture, a distinction is usually made between therapeutic and subtherapeutic uses. Public-health advocates don't object to treating sick animals with antibiotics; they just don't want to see the drugs lose their efficacy because factory farms are feeding them to healthy animals to promote growth. But the use of antibiotics in feedlot cattle confounds this distinction. Here the drugs are plainly being used to treat sick animals, yet the animals probably wouldn't be sick if not for what we feed them and the environment we place them in.