Viruses have two main components. The first is the delivery package. This is the element that is capable of delivering a payload into the interior of a cell. The second is the genetic code that hijacks a cell's normal functioning to produce more viruses. Viruses have a genetic component, either RNA or DNA, which contain instructions for building their viral envelopes. The genetic material is as vulnerable to damage as cellular DNA.

Prions are a single component. They are proteins with the same amino acid sequence as a protein normally produced by an organism. Except they have been misfolded. The geometry of the misfolded protein is such that it becomes a catalyst to reproduce the same misfolding error. There are no "brains" in them. They can arise entirely accidentally. They're a bit like the ice-nine from Vonnegut's _Cat's Cradle_, in which a crystal of a novel form of solid water that melts at a higher temperature can recruit liquid water to grow the crystal without limit. Each individual prion can misfold any number of same-sequenced amino acid chains.

And like the ice-nine, which melted at a temperature above human body temperature, the prion may be able to survive through more extreme conditions than other forms of the protein.

In the best case scenario, the prion does little more than cause a deficiency of the normally-folded protein forms in the body. In the worst-case scenario, it poisons some other process within the body in its misfolded form.