Animals raised for meat in factory farms are often genetically manipulated to grow unnaturally large and hence more profitable for the agricultural industry.
Chickens slaughtered in the United States, for example, are bred to weigh a staggering nine pounds today compared to just two pounds in the 1950s. They are bred to grow so fast, in fact, that debilitating deformities are common.
According to a study by the Center for Food Safety, over 450 animal drugs, drug combinations, and other feed additives are administered to animals to achieve increased growth and keep them alive in conditions that would otherwise kill them. Indeed, 99.9 percent of chicken and 78 percent of beef consumed in the United States come from overcrowded factory farms. Since these factory farms are filthy and packed tightly with animals, disease and infection run rampant.
Animal Ethics Project 