I had the Hong Kong flu when I was 23 and thought I was insane. My fever went to 105, it seemed as if I had two brains, one seeing crazy hallucinations and the other watching all that from a distance.
LSD’s source - ergot, a rye mold - led some to speculate that poor food conditions in the Middle Ages, where moldy bread was common, were the source of so many “visions” during that period.