Proteins do eventually break down in the environment. If you hugely multiply the number of humans and population density, you are creating a situation where pri

Proteins do eventually break down in the environment. If you hugely multiply the number of humans and population density, you are creating a situation where prion diseases can become endemic where they couldn't have been before. This is probably exactly what happened with CWD - deer overpopulation plus the random protein misfold created the conditions for a contagious disease where it didn't exist before.