The story of Helicobacter pylori is a real testament to the tenacity of medical researchers to prove their hypothesis. It took decades before the scientific world would accept that the bacteria H. pylori caused ulcers.
There are all kinds of theories—many supported by science—about what causes Alzheimer's disease. Tangles of protein called ß-amyloid (pronounced beta amyloid) plaques are prominently on the list of possible causes or, at least, contributors. An emerging theory of the disease suggests that those plaques aren't the problem, but are actually our brains' defenders. They show up to help fight an infection, and decades later, they become the problem.
