Chipotle restaurant workers fill orders for customers. The company is now sourcing some of its pork from a British supplier that uses antibiotics to treat pigs when ill. Gail Hansen, a veterinarian and longtime critic of antibiotic overuse on farms, welcomes this shift in Chipotle's stance on the drugs. Joe Raedle/Getty Images hide caption

Cattle that are grass-fed and free of antibiotics and growth hormones are seen at Kookoolan Farm in Yamhill, Ore. Don Ryan/AP hide caption

Tyson Foods says it has already reduced its use of human-use antibiotics by 80 percent over the past four years. Here, Tyson frozen chicken on display at Piazza's market in Palo Alto, Calif., in 2010. Paul Sakuma/AP hide caption

An order of McDonald's Chicken McNuggets in Olmsted Falls, Ohio. McDonald's says it plans to start using chicken raised without antibiotics important to human medicine. Mark Duncan/AP hide caption

Within a day after chicks hatch, they are sorted by sex and shipped to farms. Some will be treated with antibiotics; others will not. Dan Charles /NPR hide caption

Chicks in the Perdue hatchery in Salisbury, Md. The company says an increasing number of its chickens are now raised using "no antibiotics, ever." Dan Charles/NPR hide caption

Do these chickens look medicated? iStockphoto hide caption

Beef cattle in a barn on the Larson Farms feedlot in Maple Park, Ill. Daniel Acker/Landov hide caption

Young broilers nibble feed at a chicken farm in Luling, Texas. The Food and Drug Administration has issued new guidance on how drug companies label antibiotics for livestock. Bob Nichols/USDA/Flickr hide caption

Turkeys sit in a barn in Sonoma, Calif. An estimated 46 million turkeys are cooked and eaten during Thanksgiving meals in the U.S. Justin Sullivan/Getty Images hide caption

In recent years, pork producers have found ways to keep the animals healthy through improved hygiene. M. Spencer Green/AP hide caption

Cattle crowd inside a feedlot operated by JBS Five Rivers Colorado Beef in Wiley, Colo. John Moore/Getty Images hide caption

Piglets in a pen on a hog farm in Frankenstein, Mo. Jeff Roberson/AP hide caption

Beef cattle stand in a barn on the Larson Farms feedlot in Maple Park, Ill. Daniel Acker/Landov hide caption

A Chipotle Mexican Grill in Mountain View, Calif., in 2007. Paul Sakuma/AP Photo hide caption