Here a micrograph of human brain tissue shows buildup of beta-amyloid plaque as a circular lesion. Credit: SPL/Photo Researchers, Inc.

Researchers hope to one day treat Alzheimer's with vaccines that prevent or clear the buildup in the brain of a protein known as beta-amyloid. But an early clinical trial of one such vaccine, sponsored by Elan of Dublin, ­Ireland, was stopped in 2002 after some patients developed encephalitis, an inflammation of the brain. Later, autopsies showed that despite the inflammation, the vaccine did clear the toxic protein from the patients' brains.

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