Most people are aware that humans have bacteria that live in our intestine. Some produce Vitamin K, necessary for blood clotting. One would think that a healthy immune system would wipe out these bacteria, but actually, the resident bacteria are important. They cause the intestinal lining to produce chemicals that fight invading bacteria. When humans take antibiotics for infections, some of these beneficial bacteria are killed as well. Some people ingest probiotics available at health food stores, or eat yogurt while taking antibiotics in order to try to quickly replace the beneficial bacteria.
Evidence is growing that medical problems, such as allergies, asthma and inflammatory bowel disease, may in some cases result from the lack of proper development of the interactions between beneficial residential microbes and host tissues.
To prevent infection, most chickens are given antibiotics. Some consumers are opposed to antibiotics in their meat, because they fear it will cause increased numbers of antibiotic-resistant strains of bacteria. So there is a move among chicken producers to stop antibiotic use, but at the same time protect the public from disease.