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Light micrograph image of a neuron in the brain of a goat (Capra hircus) that has scrapie disease. This is a fatal degenerative brain disease which affects goats and sheep. It is caused by prions (red), which are misfolded proteins that induce misfolding in other proteins of the same type. Cells with misfolded proteins will die, and when this occurs in nerve cells in the brain it causes behavioural changes, lethargy, weight loss and eventual death. There is no cure presently. (KEYSTONE/SCIENCE PHOTO LIBRARY/David Schneider, US DEPARTMENT OF AGRICULTURE/SCIENCE PHOTO LIBRARY)