prions are fairly specific sequences to begin with, and then there is the folding issue that apparently makes some prions an active concern, the most concerning being those that are autotemplates and replicate themselves. commonly prions are not digestible by protease enzymes, thus accumulate like a factory floor full of broken, off spec parts.

its not good odds at all to bet on a viral mRNA fragment spontaneously creating a prion protien sequence. this is not beyond the realm of possibilities however the probability nonexistent, compared to the probability that anyone at random could have a nasty interferon mediated cytokine cascade, and sequential inflamatory response to the virus.