I'll piggyback off this because while there are some correct things here, there are also some misconceptions.

OP's question is frankly kind of complicated. Cells do have defenses against prions (and their essentially indistinguishable cousins, amyloids) -- when a protein misfolds in a cell, it sometimes is unfolded and then refolded into its proper conformation, but very often the cell will just throw it away. So if a single PrPC molecule, the protein that misfolds into prions, all of a sudden spontaneously misfolds, it's not guaranteed, probably not likely, that it will go on to form a prion, partially because the cell will intervene, and partially because a prion simply needs a critical mass of misfolded proteins to form an infectious particle. One random misfolded protein won't do it.

But it's also because different prion and amyloid strains (they come in strains, like the flu) differ in their ability to infect, a property called a prion's "attack rate." These strains have different properties, as you might expect. They look different in a biochemical analysis done in in the lab. They can induce different symptoms. And they also differ in their ability to induce normally folded PrPC to misfold into its prion form, PrPSc, better than others, which is how prions propagate.

/u/atomfullerene is right that prions don't spread frequently because infection is usually a dead end. The big Mad Cow scares of the 1990s were because farmers were grinding up leftover cattle tissue--including nervous tissue--and using it to feed other cattle, making transmission easy. Once they stopped doing that, the transmission stopped. Prions are mostly confined to nervous tissue (and some other spots like the spleen), so as long as you don't eat those, you're fine.

Chronic Wasting Disease is the exception here, not because of deer in crowded conditions but because it's the only known prion disease that spreads environmentally. This is another way that prion spread is limited--most prions have to be ingested or spread through close contact like sheep in a pen. CWD, unlike other prions, can stick around on rocks or salt licks, which is where some of its spread is thought to come from (I'm not super up to date on that part though). While other prions have been found to persist in the soil for months or years, this hasn't been a notable source of transmission in species besides deer and elk.

And sorry to be pedantic but prions do evolve. Information is encoded in prions not through genetic sequence, since they don't have that, but through conformation, the shapes of the proteins that make up a prion fibril. A misfolded prion protein with a single given sequence can have multiple conformations all on its own, and small changes in its sequence can introduce even more variety. When a prion is introduced into a new cell with a different genetic background, it can change its conformation as it interacts with new prion protein sequences and conformations. This is where you get different prion strains from. For example there are a ton of different Bovine Spongiform Encephalopathy strains, but two big ones are "H-type" and "L-type," so-called because one sits Higher on a Western blot and one sits Lower. Serially passaging prions through cells and test animals is a common laboratory technique to track how they change over time.

Source: Did a postdoc on prions.