Mad cow disease has hit the U.S. and questions about this mysterious disease abound. Here's what you need to know about mad cow disease.

Researchers believe that the infectious agent that causes mad cow disease is an abnormal version of a protein normally found on cell surfaces, called a prion. For reasons still unknown, this protein becomes altered and destroys nervous system tissue -- the brain and spinal cord.

Mad cow disease, or bovine spongiform encephalopathy (BSE), is a transmissible, slowly progressive, degenerative, and fatal disease affecting the central nervous system of adult cattle. The U.S. Department of Agriculture (USDA) has tested hundreds of thousands of cattle for BSE.

Does Mad Cow Disease Affect Humans?

A human version of mad cow disease called variant Creutzfeldt-Jakob disease (vCJD) is believed to be caused by eating beef products contaminated with central nervous system tissue, such as brain and spinal cord, from cattle infected with mad cow disease. For this reason, the USDA requires that all brain and spinal cord materials be removed from high-risk cattle -- older cattle, animals that are unable to walk, and any animal that shows any signs of a neurological problem. These cow products do not enter the U.S. food supply. The USDA believes this practice effectively safeguards U.S. public health from vCJD.

According to the CDC, four deaths from vCJD have been identified in the U.S. However, it's believed those cases were caused by consumption of meat outside the U.S.

It is important to clarify the differences between variant CJD and another form of the disease, referred to as classic or sporadic CJD. Classic CJD has no known cause and occurs each year at a rate of one to two cases per 1 million people throughout the world, including in the U.S. and countries where mad cow disease has never occurred. It is not linked to eating nerve tissue from mad cow disease-affected cattle -- both vegetarians and meat eaters have died from classic CJD. CJD most commonly affects people over 65 and is usually fatal within six months from onset of symptoms.