There are many forms of prion diseases, CJD vCJD are only a few. One thing they have in common is that the causes are unknown, only suspected.
CJD has been around for a long time. vCJD is a more recent "mutation" that effects younger folk.
The funny thing with malformed, malfunctioning prions is that they change slightly with each new host they effect. From all indications, not only do they mutate differently from species to species, but also from individuals and person to person too, depending on a number of factors.
The first CJD malformed prion was discovered in a scrapie sheep.
Click on title above to go to the prion disease center where you can learn more about it and watch an hour long vid