Junior doctor Barry Marshall was sure the medical establishment was wrong about the cause of stomach ulcers. The received wisdom was that they were caused primarily by lifestyle factors, but Marshall and pathologist Robin Warren were sure that the bacterium Helicobacter pylori was to blame.
It turned out that Helicobacter pylori was present in half the stomachs in the world — only a tiny fraction of which developed ulcers. So much for causation. Marshall and Warren did not consider that lifestyle factors might cause immune efficiency to go down, leading to increased growth of the bacterium. In a famous example of self-experimentation, Marshall ingested a giant amount of the supposedly dangerous bacterium — but, uh-oh, didn’t get an ulcer. Thanks to JR Minkel.