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Coloured Transmission Electron Micrograph (TEM) of prion fibrils in the brain of a cow infected with BSE (Bovine Spongiform Encephalopathy) or 'Mad Cow Disease'. The elongated fibrils (green) are believed to be aggregations of the protein that makes up the infectious prion. (Negatively stained. Magnification: x62'000 at 6x4,5 cm size) (KEYSTONE/EM UNIT/VLA/Science Photo Library)