Colored brain scan of a 17-year-old boy with mad cow disease. The bright yellow spots are a sign that the thalamus is damaged by diseased proteins. Simon Fraser/Science Source hide caption

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Prion protein can be infectious, spreading from cell to cell in the brain. Here four nerve cells in a mouse illustrate how infectious prion protein moves within cells along neurites — wire-like connections the nerve cells use for communicating with adjacent cells. Science Source hide caption

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Cattle feeding practices have been changed in an effort to halt the spread of mad cow disease. iStockphoto.com hide caption

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