Leeches

People used to die all the time from getting bled and cupped and purged by physicians attempting to cure ailments that might easily have resolved themselves. These doctors had theories about what was going on inside the human body that had nothing whatsoever to do with any observable outcomes, and they stuck to these beliefs even when their patients kept dying on them. When reality deviated from their theories, they declared reality to be in error. We don't so much do medicine that way these days, but governance is another story. Ideology makes smart people stupid and stupid people beasts.