I said that there is no way that the prion spreads naturally, hence once the Fore stopped cannibalizing their diseased relatives, Kuru disappeared and likewise for cows eating diseased sheep protein. But, there is an exception. The disease does appear to spread in sheep and no-one knows why. There is the mystery of Icelandic sheep. These herds subsequently came down with Scrapie.
Retroviruses especially can be extremely elusive.
They can hang around as mere shreds of RNA, but these little shreds while appearing lifeless to the biochemist can stealthily repair themselves at some opportunity and go on to multiply. And certainly shreds of DNA or RNA have been found associated with prions. Finally consider the case of Mad Horse Disease. For years no virus could be found. Then armed with new techniques Ian Lipkin at the University of California at Irvine isolated Bornavirus, the causative agent.