A random test has demonstrated that mad cow disease has infected a cow in California. This is the first case (out of four, total) of the disease in the United States since 2006 in Alabama. South Korea, an importer of U.S. beef, is not pleased.
Mad cow disease is officially known as Bovine Spongiform Encephalopathy (BSE). BSE outbreaks in the past have been attribute to feeding cows, which are natural herbivores, with supplements that include the meat and litter of other animals - including cattle.
BSE is the result of proteins, called prions, becoming misshapen and resistant to degradation. There are normally prions in the brain, deemed PrP-sen, that do not cause harmful symptoms. The "broken" prions that cause disease are called PrP-res, for "resistant." When a PrP-res is passed to a healthy organism, it comes in contact with PrP-sen, and converts the PrP-sen into the dangerous PrP-res. As the number of PrP-sen grows, the bad prions tend to cluster together, forming amyloid fibers. The amyloid fibers kill surrounding cells. As the dead cells are digested, substantial holes are left in the brain, to the detriment of the organism. Eventually, it will lead to loss of control of the body (ataxia), and subsequent death.
A timeline and explanation regarding prion-based disease can be found here.
Prion diseases like BSE (or the human manifestation called Creutzfeldt-Jakob disease) are incurable once a person is infected with a PrP-res. As well, due to the nature of PrP-res being a protein and not a living pathogen, meat from a BSE cow cannot be destroyed by heating it.
Luckily, 40,000 cows per year are selected for BSE checks. Hopefully, this is the only current case of BSE, and it can be taken care of effeciently.