Pellagra is an infectious disease that spreads among poor communities.

Pellagra seemed contagious for understandable reasons. It clustered in poor families, orphanages, prisons, and institutions, where many people became ill together with dermatitis, diarrhea, neurological symptoms, and eventually death.

Joseph Goldberger noticed a pattern that infection did not explain well. Staff caring for patients rarely developed pellagra, while residents eating the same restricted diet often did. He showed that improving food could prevent the disease and that dietary restriction could reproduce it.

Goldberger and colleagues even exposed themselves to blood, secretions, and skin material from pellagra patients without becoming ill—dramatic evidence against a transmissible pathogen.

The nutritional explanation was later traced to deficiency of niacin, or vitamin B3, and of tryptophan, an amino acid the body can use to make niacin. Diets heavily dependent on poorly processed maize and lacking adequate protein supplied too little usable niacin and made the deficiency especially likely.

Pellagra can still arise for reasons other than simple poverty or food scarcity, including malabsorption, alcoholism, metabolic disease, and certain medications.

The old infectious interpretation confused a shared exposure with person-to-person transmission: communities became sick together because they ate similar deficient diets.