The largest chicken meat producer in the United States says it will stop giving its birds antibiotics that doctors use to treat humans.Tyson Foods made the announcement April 28. It said it plans to complete the change by 2017.Health experts say overuse of the medicines is part of the reason more bacteria are becoming resistant to them.Farmers have fed livestock small amounts of antibiotics for many years.The drugs help the animals grow faster and prevent the spread of sickness among farm animals that live close together.But bacteria living in and on the animals develop and become immune to the drugs. In other words, the antibiotics no longer have any effect.These drug-resistant bacteria then enter the bodies of people who eat the birds or livestock.Then, infections in humans become more costly and difficult to treat. Sometimes, treatment does not work at all.Donnie Smith is the president of Tyson Foods. He says the company has already begun reducing its use of antibiotics.