Mad cow disease
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Though it is rare in humans, we can contract the disease through eating infected meat (but don't worry, if you die of CJD, it's much more likely you'll die of the form unrelated to cows). CJD debilitates and later kills the infected person, taking between eight weeks and a couple of years, and is 100% fatal.
The brains and spinal cords of infected animals are considered more infectious than other parts, and there are thus "rules against beef processors using the brains or spinal cords of the animal to make food products".
Because of it, you still can't give blood in the USA if you were in the United Kingdom for more than six months total from 1980 to 1996. 
Our best theory as to the mechanism behind Mad Cow Disease, and other transmissible spongiform encephalopathies, is that it's not caused by a virus or a bacterium or a parasite or a congenital condition, but instead by a misfolded protein called a prion.
Prions are fascinating molecules. Like all protein molecules, they are chains of amino acids. In general, every protein molecule in your body does its job because of its shape; the way a given amino acid chain is "folded" in three-dimensional space determines what kinds of other molecules it can capture, latch on to, twist apart, etc.. Normally, every amino acid chain will fold in one specific, predictable manner as it's being assembled from one end to the other. However, in a few cases, the same amino acid chain can be folded into a very different shape after it's been assembled, if it's acted upon by the proper outside force. This will make the molecule incapable of performing the duty its old shape had.
Prions are, amino-acid-for-amino-acid, identical to a nerve tissue protein called PrPC. However, they're folded in a radically different manner, such that instead of performing the normal duties that a PrPC molecule would, they instead act as catalysts to re-fold PrPC into new prions. Thus, one prion will, when introduced into neuronic tissue (such as brain tissue), create more prions, and these will in turn create even more prions -- all the while robbing the tissue of the PrPC it needs to function properly.
This quasi-self-replicating behavior has led some to speculate as to whether prions should be considered a form of life.
Frustratingly, prions don't break down in the presence of acid or heat. This means that neither cooking, nor the digestion process, can remove prions from infected cow brains.
Cows were not the first species to be infected by prion diseases. The sheep disease Scrapie is now known to be very similar to mad cow disease. It first reached epidemic proportions in cows, however, due to the practice of grinding up the undesirable parts of a slaughtered cow (including the cow's brain) and feeding this protein meal to other cows. Since the alarm bells were raised for mad cow disease, this practice has been discontinued.
- Bovine spongiform encephalopathy from Wikipedia
- The Official Mad Cow Disease Home Page
- What’s Mad Cow Disease?
- Formerly New Variant, but the new was dropped once fear of the whole UK population dying horribly got old.
- Risk of death from variant Creutzfeld Jacob Disease (vCJD) and CJD in the UK
- What is Mad Cow Disease?
- FDA Website
- Mad cow disease was brought to Earth by aliens - urban myth
- Alien invasion and mad cow disease