The problem with that is there are way more pathogenic viruses originating in animals than prion diseases, and (probably?) way more virology labs than labs studying prion diseases. That makes both factors in the equation much larger in the case of viruses than prion diseases. Just off the cuff, I'd guess that would make the overall probability of a zoonotic virus emerging near a city with a virology lab at least an order of magnitude greater than your prion disease scenario.