What is a prion?
Simply put, a prion is a protein like any other, and it does its job, just like any other. The human body is essentially a lot of proteins floating in water, each doing the only thing they can possibly do. They do their job because of the way they are folded, and their shape and the charges on their various locations then perform the task automatically.
The human prain has a certain protein called PrPC, which does its job without any problem normally. it's related to the structural integrity of the brain cells.
This protein, however, through some immense combination of chance, has an alternative configuration, meaning it can be folded in a second (and very unusual) way. Because it is folded differently, it does a different job, and is called PrPSc instead of PrPC.
The job it does is turn other PrPC proteins into PrPSc proteins.
Since the PrPSc does not properly do the brain function it's supposed to, and instead forces other proteins to not do their jobs, it is an infectious agent, called a Prion (or Proteinaceous Infectious Particle, pronounced "pree-on"). Hypothetically, any protein that forces other proteins to misconfigure and cause a disease could be a prion, but this brain prion is the only one known to science to actually have this happen.
The misfolded prion has a much higher level of stability than the normal protein and so cannot be folded back. It is also not properly broken down with detergents or even pepsin in your stomach! And since it is a protein, the immune system cannot target it like it would a virus or bacteria. The immune system CAN clear the plaques, which are the symptoms of prions, but the individual proteins fly under the radar.
The symptoms don't show up for years, but gradually the brain begins to lose shape and take on a spongy form. The victim begins to slowly lose grip on reality, motor control, all that good stuff. Eventually, complete insanity takes over, followed by death. 100% kill rate on every prion disease known to man: kuru in the natives of Papua New Guinea, CJD in the elderly, chronic wasting disease in deer, scrapie in sheep, and Mad Cow's Disease in cattle.
There is no cure, and it is invariably fatal. It can also be passed across species boundaries at reduced infection rates (cow prions can give us Mad Cow Disease, sheep prions can give us Scrapie, but it's not as efficient as when humans give each other nvCJD or kuru). It is also nearly impossible to detect, and has a very, very long incubation period (many years).
So, why do they call me Prions?
Well, to be fair, I called myself it first. I've always been an admirer of something so invisible, so deadly, so indestructible, and so mysterious that an entire new branch of science had to be established just to confirm it existed.
So when they call me Prions, they pay me high praise as being something as evil, invincible, and unstoppable as a protein that makes you go insane and die.