[ctdonath]

Bacteria can do things on their own (say, swim in water). Viruses can't do anything without a host's reproduction engine (like a program which can't do anything without a computer to run on). Prions are misshapen proteins which tend to damage other proteins such that the result is the same misshapen, and thus duplicating, form.

[PascalsMugger]

I think what was meant is that on the grand scale of "aliveness" viruses are more alive than prions. Viruses have DNA, they actively seek out host cells to hijack so they can reproduce, etc. Prions are much simpler and aren't alive at all. They're just accidentally self-reproducing patterns, basically. They don't have DNA, they don't make any specific effort to reproduce, they just "happen" from weirdly broken matter bumping together.

[hodwik]

Viruses don't make any specific effort to reproduce, they too are just accidentally self-reproducing patterns.