Having some unknown person(s) spreading Prion diseases around that cannot be detected until way after it happens would be close to the perfect terrorist attack imo. You could probably keep the attack going for at least a year before authorities get their act together, all the while people would be scared shitless as it would be very hard to know how the original group got infected a year earlier if you choose the right product to contaminate. (Not even mentioning the difficulty finding who did the contamination even if the specific product is found)
We also don't really know what the population penetrance of prion is. It's possible, for something like british mad cow disease, basically everyone got exposed, but only some people got sick due to a random high infectious load, or a weakened capability of the body to clear the prion...
There are proteases in the body that are capable of clearing amyloid forms (insulin-degrading enzyme, e.g.) and it would be hard to believe that for a sufficiently small dose it wouldn't get cleared before it got to the brain.
strange, I just looked it up, but no one has investigated if increasing IDE expression in mice has a protective effect against prpSc infection (when the infection is not administered directly to the brain)... If there are any prion researchers here... Might be an interesting experiment.