A rare neurological disease
that was unfortunately made less rare
recently when unsuspecting British citizens
ate meat from cattle
infected with Bovine Spongiform Encephalopathy
Classic CJD is not caused by a bacteria or a virus, but by a prion, which is a simple protein found in the brains of birds and mammals. Normally harmless, a mutated form of the protein can build up in neurons and eventually cause brain damage, and leave the brain pitted with holes. This causes a progressive loss of mental function and motor control, until the victim dies. Until this decade, this disease was almost always either inherited or sporadic (meaning that it was unclear how the defective prion got in the brain), with only 1% of cases caused by infection.
The new version tends to hit younger people (median age:28), and has a more delayed onset of neurological problems starting with ataxia and progressing to dementia and myoclonus later on. There's no definitive proof yet, but the new version was most likely the result of eating beef from cows that had the mutated prion.
There is no cure for CJD, but if you want to avoid getting it, then don't eat cows that eat cows.
Many thanks to both britannica.com and cdc.gov