I remember the issue was prions. Something even smaller than a virus. Basically a chemical that causes the brain to start making copies until it makes the victims crazy.
A prion is a protein that is misfolded.
The proteins that can be misfolded into a prion form are present in everyone. But they are folded correctly. In the vast majority of people, they will never misfold.
I think that some mutations make the protein more likely to misfold. And the misfolding is contagious, as if proteins are using misfolded proteins as a folding guide.
When these proteins are misfolded, they can no longer be enzymatically destroyed by the cell. Most proteins are destroyed within hours of being made. But the prions just stick around. This probably has something to do with the pathogenic process.