Transmissible spongiform encephalopathy, also known more colloquially as mad cow disease is a non-contagious disease transmitted by malformed proteins (prions) found in the brains of infected animals, generally rumiforms such as cows and sheep (where the disease is known as "scrapie"). The disease can only be transmitted by eating infected tissue and cannot be passed from person to person.
Once inside the body, the prions gradually destroy brain tissue, litterally punching small holes through every part of the brain. Brain function can decline either slowly or quickly depending on the progression of the disease with higher functions (such as reasoning and memory) disappearing first, and other brain functions disappearing gradually until death.
The disease is very rare, with fewer than a handful of cases worldwide every year. Eating beef does not seem to be a major risk factor and even persons who appear to be vegetarians may contract the disease. Cattle populations are checked regularly for signs of the disease and infected cows are destroyed and are no longer allowed to re-enter the food chain (they were once processed into cattle feed, which often reinfected animals). In addition, procedures to slaughter cattle take care to ensure that the head and spinal cord are removed before the rest of the carcass is processed, lessening the chance of contamination.