Once upon a time scientists worked on genetically modifying animals and plants to get bigger and better consumable yield.
The scientists made some headway in making chickens bigger and fatter, but the effect somehow plateaued. The investors weren't satisfied.
One scientist was sifting through his old notes from college when he came across an article about a virus that caused stem cells to mutate specifically into fat cells in certain animals. He got the brilliant idea to modify the virus to infect chickens, effectively doubling their size.
Even more brilliant was that the virus would not need to be given to each chicken because it was an airborne virus and spread easily across chickens, especially if they were packed together in confined spaces.
Unfortunately the virus mutated and infected people who ate chicken.