Answer:
Cows become infected with the prion that causes bovine spongiform encephalopathy (BSE) by ingesting it. The prion is a misfolded protein found primarily in the central nervous system (brain and spinal cord). When a cow died with BSE and was rendered (basically cooked down into unrecognizable amino acids), the prion was not broken down. This rendered bovine protein was then added back to cow's rations to provide them a balanced nutritional diet - but it also infected them with the prion.