Since the 1950s, the agricultural industry has used antibiotics as a precautionary measure to prevent widespread infection in the crowded, restrictive settings of a food animal farm. Antibiotics are readily available, low cost, and promote profitable weight gain in food animals compared to other capable forms. Approximately 80% of all antibiotics sold in the United States can be traced back to agricultural usage and many overlap with the antibiotics used to treat human illnesses. The World Health Organization classified several growth-promoting antibiotics utilized by food corporations as critically important to human medicine. The FDA does not strictly regulate the use of antibiotics for agricultural purposes.