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DISEASES such as BSE in cows, scrapie in sheep and vCJD in humans happen when deformed prion proteins infect the brain. Scientists weren’t sure how the infection spread through the brain, but they have now discovered that all it takes is physical contact between cells. Nennaya Kanu and her colleagues at University College London took mouse brain cells infected with scrapie and grew them in a nutrient solution together with healthy cells. They found that when normal prions on the surfaces of healthy cells came into contact with the deformed prions on scrapie-infected cells, they changed to match the shape of the abnormal prion. This finding should allow scientists to look for ways to prevent infection, says Kanu (Current Biology, vol 12, p 523).