Unlike bacteria or viruses, prions appear to become infectious merely by assuming an altered shape. They then transmit their characteristics via protein-protein interactions, making otherwise normal prion proteins also adopt an abnormal shape. These abnormal proteins become insoluble, which¿in mammals, at least¿leads them to clump together and create brain-cell-killing plaques.
In order to understand how prions bridged the biological gap between different species, Jonathan Weissman and graduate student Peter Chien stitched together segments of prions from two different species of yeast. They found that the resulting hybrid could become infectious in both original kinds of yeast¿exactly the sort of ability needed to cross over into a different organism.