Given the conditions in which animals are forces to live in the factory farming, the only way to keep them healthy is the use of antibiotics (for instance, in the USA 80% of the sold antibiotics turn up in animal feed).
Since the 1950’s farmers have started to give antibiotic growth promoters (AGP) to livestock, in order to make animals get fat faster and antibiotics to treat illness to those specimen that were sick.
However, an excessive use in these drugs is leading to the evolution of “superbacteria” which are resistant to antibiotics and can be transmitted to human beings.
The European Union is considering the risks linked to the use of antibiotics in the stockbreeding. It banned the non-therapeutic use of drugs and even some American enterprises stopped to use them.