And it seems very few proteins appear to be significant problems.

The most famous is the prion protein which can misfold in ways to cause a variety of contagious diseases. Like mad cow disease, chronic wasting disease, scrapie and in humans CJD and vCJD, fatal familial insomnia, Kuru, GSS.

Perhaps because misfoldings of the prion protein can convert others but why is it all affecting that same protein? Always baffled me why aren't other/many proteins suspitible to becoming a prion?

There are others we call "prionoid" because they can have shades of the catetrosphic misfolding prion can.