But prions don't actually reproduce--they already exist in our brains. All mammals have prion proteins in their brain. The propagation starts when one of these normally folded one misfolds, or a misfolded one gets into the brain from some other source. Then it causes the existing proteins to misfold. No new molecules are created in this process. In fact, a disease form of a prion is the exact same molecule as a healthy form. It's just a change in shape. Viruses don't hit on all the hallmarks of life, and prions hit even fewer.