Many of the animals who live in slaughterhouses are given antibiotics, a lot of antibiotics, so that they don't get sick living in this awful crowded environment. In fact, some estimates state that close to 50% of all antibiotics in the USA are used on animals being raised for food. Thus, when you consume the meat from these animals, you consume some of the residual antibiotics, which may in turn weaken your immune system in the long run.