First recognized in the 1980s, mad cow disease -- or bovine spongiform encephalopathy (BSE) -- causes the brains of cattle to waste and become spongy and pockmarked.

NATURE's six-part series TRIUMPH OF LIFE presents the remarkable story of evolution and its survivors.

NATURE's six-part series TRIUMPH OF LIFE presents the remarkable story of evolution and its survivors.

NATURE's six-part series TRIUMPH OF LIFE presents the remarkable story of evolution and its survivors.

NATURE's six-part series TRIUMPH OF LIFE presents the remarkable story of evolution and its survivors.

NATURE's six-part series TRIUMPH OF LIFE presents the remarkable story of evolution and its survivors.