Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is a fatal, neurodegenerative disease of cattle, which infects by a mechanism that shocked biologists on its discovery in late 20th century. The disease appears transmissible to humans.

While never having killed cattle on a scale comparable to other dreaded livestock diseases, such as foot and mouth and rinderpest,  BSE has attracted wide attention because people assume humans can contract the disease, but it has never been proven that BSE has any link to variant Creutzfeld-Jakob disease (vCJD), sometimes called new variant Creutzfeld-Jakob disease (nvCJD), a human brain-wasting disease.

Infectious agent
Unlike other kinds of infectious disease which are spread by microbes, the infectious agent in BSE is a specific type of protein. Misshapen ("misfolded") prion proteins carry the disease between individuals and cause deterioration of the brain. BSE is a type of transmissible spongiform encephalopathy (TSE). TSEs can arise in animals that carry a rare mutant prion allele, which expresses prions that contort by themselves into the disease-causing shape.