Almost two years ago the Obama administration banned the slaughter of "downer" cows, cows that exhibited neurological dysfunction. The move was aimed at lowering the chance of infecting consumers with "Mad Cow Disease," more appropriately known as Creutzfeldt-Jakob Disease.
The disease attacks the brain, destroying a person's ability to think and speak, destroying his memories and personality. In effect, the symptoms are hardly different from dementia or Alzheimer's.
While the ban was a needed move in the right direction, the USDA and other government agencies are not going far enough. "Downer cows" are cows that have hit the very last stages of mad cow disease—cows whose brains are swimming with the particle responsible for this disease: the prion.
A prion is a protein that has the unique ability to take other proteins and turn them into its own shape. Prions are like zombies (or for you Trek fans, the Borg) who take normal human beings and turn them into one of them. Eventually, so many of these proteins accumulate in the brain that it results in brain damage. If your brain cells (neurons) are the pipes that deliver information, the plaques that form from the proteins clumping together are like sewage stopping up the pipe. In this case, the pipe can burst: those protein clumps can destroy entire brain cells.
This disease spreads because our beef industry is insane enough to feed cows the inedible remains of other cows. The parts that humans are loathe to eat—such as the brain and spinal cord—are ground up and fed to cows to save money on feed. Of course, an infected cow's brain and spinal cord are full of prions. These prions go on to infect more cows, creating more of themselves, and so forth. Unfortunately, a cow can still seem somewhat healthy and be infected with a lot of these prions. That's why banning "downer cows" just doesn't go far enough.