Alison Abbott in Nature:
Two years ago, immunologist and medical-publishing entrepreneur Leslie Norins offered to award US$1 million of his own money to any scientist who could prove that Alzheimer’s disease was caused by a germ. The theory that an infection might cause this form of dementia has been rumbling for decades on the fringes of neuroscience research. The majority of Alzheimer’s researchers, backed by a huge volume of evidence, think instead that the key culprits are sticky molecules in the brain called amyloids, which clump into plaques and cause inflammation, killing neurons. Norins wanted to reward work that would make the infection idea more persuasive. The amyloid hypothesis has become “the one acceptable and supportable belief of the Established Church of Conventional Wisdom”, says Norins. “The few pioneers who did look at microbes and published papers were ridiculed or ignored.”
In large part, this was because some early proponents of the infection theory saw it as a replacement for the amyloid hypothesis. But some recent research has provided intriguing hints that the two ideas could fit together — that infection could seed some cases of Alzheimer’s disease by triggering the production of amyloid clumps. The data hint at a radical role for amyloid in neurons. Instead of just being a toxic waste product, amyloid might have an important job of its own: helping to protect the brain from infection. But age or genetics can interrupt the checks and balances in the system, turning amyloid from defender into villain.