Apparently, ranchers have been known to feed dead and/or diseased cattle, sheep, pigs, etc to their cows mixed with grass, hay, etc. These prions are then in the meat. Humans eat the meat and the prions are in our bodies but basically dormant. If we become sick or our immune system is over-burdened or we are immune suppressed...the prions take over and you can get full blown Creutzfeldt Jacob disease which attacks the brain.