Alzheimer’s disease seems to spread like an infection from brain cell to brain cell, two new studies in mice have found. But instead of viruses or bacteria, what is being spread is a distorted protein known as tau. The surprising finding answers a longstanding question and has immediate implications for developing treatments, researchers said. And they suspect that other degenerative brain diseases like Parkinson’s may spread in a similar way. Alzheimer’s researchers have long known that dying, tau-filled cells first emerge in a small area of the brain where memories are made and stored. The disease then slowly moves outward...