Pellagra usually comes from eating a high corn/sorghum diet without proper nixtamaliziation (I really hope I spelt that correctly), the alkalisation of the food before cooking. While some chronic digestive diseases, alcoholism and a few other causes can also lead to it, malnutrition is the main cause and neither of these causes come upon an otherwise healthy looking child overnight. While I might not rule out that someone is idiotic enough to feed their kid only corn and sorghum it is rather unlikely and diarrhoea is one of the rather late symptoms.

(I transferred a five year old with major liver and some cerebral damage after the parents kept the poor kiddo on a “barley, corn, yeast, self produced apple juice and full grain” only diet for a year…one can imagine what that leads to)