Mad cow disease, officially known as bovine spongiform encephalopathy (BSE), is a cattle disease that attacks the central nervous system. That is, it moves through the spinal cord and brain, literally killing brain cells and leaving holes in the brain. The visible effects of this disease are erratic, or insane, behavior in the infected cow, hence the name mad cow disease. Mad cow disease can be transmitted to humans who eat contaminated meat from an infected cow. This makes the disease doubly threatening - it endangers the food supply as well as individual human lives.
In the late 1980s, BSE was first seen in British cattle. It is thought to have entered the cattle population through a similar disease affecting sheep. At that time, byproducts from slaughtered sheep and cows were added to cattle feed to increase the amount of protein they consumed, and this practice is believed to have led to the disease in cattle.
Humans who eat contaminated beef acquire a related disease, called new variant Creutzfeldt-Jakob disease (nvCJD), a neurological disease with a progression and effects similar in humans to mad cow disease in cattle. Over 150 people in Europe, mainly in Britain, were infected and died of the human form of the disease. Because the infectious agent that causes BSE can lie dormant for a number of years, putting the pieces of the puzzle together took some time, but it is now believed the disease is transmitted via a misshapen protein called a prion. This agent exists in nervous system tissues, so meat that does not come from these tissues - that is, from animal brain and spinal tissue - is probably safe.
Mad cow disease has been controlled over the past several decades by eliminating the use of nervous system tissues in cattle feed and ground meat products such as hamburger and sausage. There have been several cases of mad cow disease in the US recently, but they were isolated and are thought to have been contained. There is currently no treatment for the disease or the human nvCJD. In areas where the food chain is suspicious, avoid eating ground meat or dishes that include animal brains.