The severe dermatitis associated with pellagra, courtesy NIH
Pellagra or niacin deficiency is a disease of malnutrition. Once fairly common, it's becoming more rare as its dietary causes become better understood and niacin is added to many foods. Although it can have other causes, it's almost always caused by a lack of niacin in the diet.
Ironically, although pellagra is usually typical of a diet based on corn (maize), where maize is a traditional dish, it's prepared with lye to release its dietary niacin. However, as corn spread from the Americas to become a staple of diets everywhere, the general methods of preparation meant the maize was niacin deficient.
It was first recognized in Spain in the 18th century, although its causes were not clear. As corn became a dietary staple in the American South, the disease became pandemic, with tens of thousands of cases arising every year, and often well over a thousand in a single state in a single month at times. A study by the U.S. Public Health Service was commissioned based on the belief the disease was due to a toxin carried in the corn. Instead, the study found that pellagra always occurred when a person was given an all-corn diet and the physicians induced pellagra in prisoners in just two weeks to prove the point. The prisoners returned to health once fed a variety of vegetables. Brewer's yeast, which was common and cheap, was shown to eliminate pellagra, but in most cases a balanced diet of different staple foods staved off the disease. It was not until 1938 that niacin was shown to be the key deficiency in persons who suffered from the disease.
Because pellagra was common in children as well as adults, it was often mistaken for intellectual disability. Many children living in institutions developed the disease.
At present, the disease is still common in poorer areas. In the United States, it's limited to alcoholics, the severely malnourished, and people who refuse food.
The classic symptoms of pellagra are the "four Ds" - diarrhea, dermatitis, dementia, and death (usually 3-5 years after symptoms appear). It can also present with sensitivity to sunlight (which brings on related dermatitis), aggression, skin lesions, insomnia, muscle weakness, confusion, and ataxia. Although most suffers will respond to a diet rich in niacin, Nicotinamide can be used in severe cases.