Pellagra is the deficiency disease of niacin (vitamin B3), remembered by the “four Ds”: dermatitis, diarrhoea, dementia, and death. It ravaged populations dependent on untreated maize, which contains niacin in a bound, unavailable form and is poor in tryptophan, the amino acid the body converts to niacin. The American South suffered epidemics into the early twentieth century until Joseph Goldberger proved the dietary cause and niacin fortification of flour followed. Traditional Mesoamerican nixtamalization, cooking maize with lime, releases the bound niacin, which is why the disease never plagued its homeland. Dermatitis appears on sun-exposed skin, diarrhoea from gut inflammation, and dementia from neurological damage. Pellagra’s conquest through fortification stands among public health nutrition’s landmark victories.