Antibiotics are used on a tremendous scale in factory farming to suppress the spread of diseases amongst animals which are housed so densely. Whilst this means that the animals are generally disease-free, the advent of a new resistant strain of infectious organism can wipe out entire houses of animals

Human medicine is the major user of antibiotics, but not by much. Farmers actually use more than 40% of all antibiotics sold in the United States. About 80% of the antibiotics used in agriculture are added to poultry, hog, and cattle feed, not to treat sick animals but to promote growth and prevent disease. This indiscriminate and non-essential use of antibiotics in agriculture dangerously increases the possibility that these antibiotics (and other closely related ones) will be ineffective when needed to treat people. Overuse of antibiotics in agriculture has led to serious antibiotic-resistance problems in foods. Strains of Salmonella and other disease-causing organisms found in raw and undercooked meat are increasingly resistant to several antibiotics. One strain of Salmonella that is resistant to five different antibiotics increased from 0.6% of specimens tested in 1980 to 34% in 1997