It’s a certain protein that got misfolded, if it comes in contact with more of the original protein it causes that to also get misfolded. It can spread kinda similarly to a virus, via bodily fluids, animal-animal contact, in contaminated food/water, and by eating tainted meat in this case. Even if the deer was asymptomatic they can still spread the disease. The way the protein is folded causes it to be resistant to degradation so it can lay dormant for years or maybe decades in the environment. It causes horrifying symptoms causing deer to almost turn into zombies as their bodies waste away.