When Europeans colonized the Americas, they found corn, an easily-grown and inexpensive grain. Eventually, many of the poorest people in Europe were eating little besides corn, made into polenta in Italy, and began to suffer from a disease called pellagra. For hundreds of years, no one knew what caused pellagra, but some suspected it was caused by a fungus or insects associated with corn. Only in the 1920s did they realize it was a nutritional deficiency, and in the '30s it was found to be a lack of niacin (vitamin B3). The poor folks who consumed mostly polenta suffered from skin rashes and diarrhea, and if it went untreated, they developed dementia, called pellagrous insanity.
During those hundreds of years, Italian sufferers could end up at San Servolo or San Clemente, two islands off of Venice with hospitals for the mentally ill. Treatment of these inmates varied according to their social status and the medical philosophy of those in charge of the hospitals. It took way too long for authorities to figure out why an improved diet would "cure" individuals, only for them to return later after eating little besides polenta in their home towns. Read about the mental hospitals of San Servolo and San Clemente at Smithsonian.
(Image credit: Kasa Fue)