The causes of variant and classical forms of CJD seem to be different. Variant CJD is associated with eating meat from cows infected with BSE. Classical CJD is an older form of CJD, which usually appears spontaneously without any apparent cause and has not been linked to eating contaminated meat. It may be associated with genetic factors, and in some cases it may be caused by contaminated medical equipment.
The link between vCJD and eating infected meat was discovered when 23 people in Britain developed CJD in the mid 1990s, about 10 years after Britain was struck by an epidemic of bovine spongiform encephalopathy in 1984 to 1986. The total rate of CJD was only about 15% higher than usual, but these victims stood out because they were young (the average age was 29) and had different symptoms than were thought to be typical of CJD. Because of these unusual features of this variant CJD, scientists started to investigate. It was already known that some neurodegenerative diseases (diseases that destroy the brain or nervous system) can be caught by eating animals that had the disease.
Scientists began to suggest that British cows had been infected with BSE because they had been fed the rendered remains of sheep. Sheep suffer from a neurological disease called scrapie, so-named because the affected sheep would try to scrape off their coat of wool by rubbing against fence posts and trees. It was then that scientists started to explore the theory that if cows could catch BSE by eating sheep who had scrapie, then maybe people could catch variant CJD by eating cows who had BSE. Evidence began to accumulate that confirmed the ability of the disease to cross the species barrier when cases of degenerative neurological disease started to emerge in zoo animals in Europe. These animals had also been fed remnants of sheep.
Previous tests of scrapie and CJD in mice and monkeys showed that it took about 10 years for symptoms to appear after eating nerve tissue from an infected animal. This matched the British timeline perfectly.
There was a problem in the theory, however. No one had found a bacterium or virus that caused any of these diseases. In fact, scrapie-infected tissue was still known to be infectious even after being exposed to radiation that would destroy DNA, and thus kill any bacterium or virus. Finally, it was discovered that a protein called a prion seemed to be causing the damage.
Proteins are long molecules made of smaller parts called amino acids that are folded up into particular shapes. A prion is folded differently from the normal protein. It also has the ability to make normal proteins that touch it fold the same abnormal way, leading to a domino effect, causing abnormal protein folding that spreads from one protein to the protein beside it. This is a completely new form of disease, caused by a protein that's not alive but is capable transforming the proteins around it.
It now seems that scrapie, BSE, and CJD (classical and variant) are all caused by prions transmitted from one affected brain to another brain.
At this time, it is not believed that prions are spread through casual contact, kissing, or sexual intercourse.