Between 1906 and 1940 pellagra struck roughly three million Americans and killed more than 100,000, progressing through the four Ds: dermatitis, diarrhea, dementia and death. The episode explains it as a deficiency of niacin, vitamin B3, and of tryptophan, and traces it back to Gaspar Casal’s 1735 description of Asturian leprosy in Spain and the outbreaks in northern Italy, where more than 100,000 people had it by the 1880s. The common thread was corn. Indigenous cultivators in the Americas had long soaked kernels in lime water, a process called nixtamalization that frees the niacin, but Europeans took the seeds without the method.

In the American South the disease became known as spring sickness. Sharecroppers trapped growing cotton lived through winter on cornmeal and molasses, and the Beall degerminator of 1901 stripped what little nutrition the milled corn had. Assigned in 1915, Joseph Goldberger noticed that asylum inmates fell ill while staff did not, cured patients by improving their meals, and then induced the disease in healthy prisoners on a restricted diet.

• The Casal collar, the sunburn-like band of dermatitis around the neck
• Orphanage data showing that only children aged six to 12, too old for milk and too young for work rations, were getting sick
• The Rankin Prison Farm trial: 11 volunteers promised pardons, with six developing lesions after about five months
• Conrad Elvehjem’s 1937 black tongue research in dogs and Tom Spies proving niacin cured human pellagra
• Why women historically got pellagra at about twice the rate of men, and where the disease still appears today