> Antibiotic-resistant strains of bacteria enter the general human population generating a significant health crisis.

From the wikipedia link above: "However, any antibiotics deemed medically important to humans by the CDC are illegal to use as growth promoters in the U.S. Only drugs that have no association with human medicine – and therefore no risk to humans – are allowed to be used for this purpose."

(I am no expert, so I don't know whats the bottom of this.)