The disease turns one's brain to sponge. Actually it crystallized some proteins in the brain and slowly causes the brain to cease functioning. It is supposed to be rather painful and accompanied by loss of bodily functions and inability to sleep. The "infectious" agent is believed to be a small protein called a
prion Getting this in the brain causes crystallization of similar proteins, like dropping a crystal into a saturated sugar solution. Prions are not inactivated by cooking.
The tendency to CJD runs in families. There are about 300 spontaneous cases per year among people not at risk. No treatments.
The Mad Cow variant seems to develop much more rapidly. The disease is similar to scrapie in sheep and kuru among Bornean canibals. Similar diseases cause wasting in elk, moose, and mice. The Mad Cow variant seems to be the only one that has jumped species.