From 1900 until the 1930s, pellagra, a disease later discovered to result from a deficiency of the vitamin niacin, was common among poor cotton farmers in the United States whose diet consisted mostly of corn, the least costly food they could buy. Corn does not contain niacin in usable form. Curiously, during the Depression of the early 1930s, when cotton's price collapsed and cotton farmers' income declined, the incidence of pellagra among those farmers also declined.
Which of the following, if true, most helps to explain the decline in the incidence of pellagra?
【选项】Until the discovery of pellagra's link with niacin, it was widely believed that the disease was an infection that could be transmitted from person to person.