I find prions unbelievably scary too. What reassures me is that they've had about 3 billion years to wipe out life on earth and haven't done it yet. Which means there must be some natural limits on their spread we don't know about yet.
If, say, oceanic fish were susceptible to some easily transmitted prions, you would have a worldwide epidemics in no time. Fish form a crucial part of almost all food chains on the planet.
The Indians still had trade going with the middle east during the black plague, but by some magic it doesn't seem to have made it by land or sea