Ever since 1906, when German physician Alois Alzheimer described the degenerative brain disorder that bears his name, doctors have argued about what exactly causes the disease. Dr. Alzheimer carefully noted two main features of the autopsied brains of his patients: the dense clumps, or plaques, of protein that showed up where nerve cells should have been and the tortured tangles that many of the neurons had become. But whether the plaques or the tangles triggered the illness or they were both just the most visible effects of some other, more obscure process no one could say for sure.
Now the century-old...