First, it is not known for sure what causes it. Other posters mention the theory that it is a protein. That's not strictly correct - the theory actually refers to the agent as a prion.
However, this is ONLY a theory. The agent is undetectable, can survive temperatures in excess of 2000 degrees celcius, and is capable of moving through the lining of the stomach. Intact proteins generally can't do any of these.
A Professor of Microbiology, at Leeds University, who has studied Mad Cow Disease more than any other person, and was the first to identify that it was transmissable to humans, has also said that up to 50% of the population of Great Britain could be vulnerable.
This does not mean Black Death II, but rather that this needs to be taken a bit more seriously than it is.
(Further, as the agent DOES survive extreme temperatures, it is entirely possible that the mass incineration of cattle in the 1990's has led to contamination of the surrounding areas with this deadly agent.)
Nobody knows - for sure - what the incubation period is, in humans. For cattle, the -minimum- is fairly well established, but as cases are still happening, the maximum is completely unknown. For humans, neither limit is known, or can even be guessed at.
The probability of half of England having their brain turn to swiss cheese in the next few years is extremely small, but it's not zero. What it actually is, is a mystery. But at least we know it's not zero.
IMHO, this is something the British should be researching. Forget that gravitational telescope in Scotland - it's not going to see anything. Forget genetic engineering scorpion caterpillars. These experiments really don't achieve anything, beyond some headlines. Concentrate the money on the REAL threats, such as the new variant of CJD, the risk of active CJD agent in and near the sites used for burning, etc.
(Mind you, Britain has plenty of other unusual diseases that could do with better understanding, especially by the public. Flesh-eating bacteria that can render an entire human into a skeleton in hours, for example, make scary headlines. Fact is, it can be easily cured, if treated soon enough. But if nobody knows the symptoms, then who is going to identify it?)