The infection, called bovine spongiform encephalopathy or BSE, turns the brains of animals spongy. Consuming infected meat can cause a related human disease called variant Creutzfeld-Jakob disease, a fatal brain-wasting disorder. Unlike diseases caused by bacteria or viruses, BSE is caused by a self-replicating protein called a prion. 
 

Links 

• US FDA site on BSE - includes fact sheets on "Mad Cow Disease," information on food safety, precautionary measures that consumers can take, and more.
• United States Dept. of Agriculture (USDA) - 
• Food Safety and Inspection Service (USDA) on BSE - Best source on the web for reliable and complete information.
• World Health Org: Bovine Spongiform Encephalopathy (BSE) Fact Sheet -  includes fact sheets on the transmissible spongiform encephalopathies BSE and vCJD, their on public health, and worldwide surveillance and control efforts. 
• Creutzfeldt-Jakob Disease Foundation, Inc. -  promote the research, education, and awareness of the disease, as well as to provide support services for persons affected by CJD.
• Yahoo! News Service on BSE - latest news coverage.