Chicken vaccines merged to form live viruses and caused outbreaks of irony

"By using multiple vaccines in the same animals, the farmers acted as matchmakers for the intercontinental viruses. The weakened Australian and European strains infected the same cells, “mated” with each other, and shuffled their genetic material (the technical term is “recombined”). Each individual strain was weakened by different mutations – different changes to their genes. When the strains merged, and their genes shuffled, they produced hybrids that lost all of these mutations, and could resume their normal virulent lifestyles. They proved to be far better at causing disease than either of their parents."