CJD belongs to a family of diseases called "prion disease" [pree-ahn] caused by abnormal "prions" – infectious, misshapen versions of a normal protein. In prion disease, the normal prion protein folds into an abnormal shape, which prevents it from doing its job and being recycled by the cell. These abnormal prions can then infect other normal prion proteins and induce them to change shape as well. Prions cause a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE or "mad cow disease") in cattle and scrapie in sheep.