The disease is one the very name of which was almost unknown in the United States until within the past decade. It has usually been regarded as tropical, though it occurs commonly in Italy and in various parts of Europe. Now it is known that it not only occurs quite generally in the United States but that it is spreading. Lavinder (1911) says that "There are certainly many thousand cases of the disease in this country, and the present situation must be looked upon with grave concern." It is not within the scope of this book to undertake a general discussion of pellagra. The subject is of such importance to every medical man that we cannot do better than refer to Lavinder's valuable précis. We can only touch briefly upon the entomological phases of the problems presented. The most commonly accepted theories regarding the etiology of the disease have attributed it to the use of Indian corn as an article of diet. This supposed relationship was explained either on the basis of, (a) insufficiency of nutriment and inappropriateness of corn as a prime article of food; (b) toxicity of corn or, (c) parasitism of certain organisms--fungi or bacteria--ingested with either sound or deteriorated corn. In 1905, Sambon proposed the theory of the protozoal origin of pellagra and in 1910 he marshalled an imposing array of objections to the theory that there existed any relationship between corn and the disease. He presented clear evidence that pellagra existed in Europe before the introduction of Indian corn from America, as an article of diet, and that its spread was not _pari passu_ with that of the use of corn. Cases were found in which the patients had apparently never used corn, though that is obviously difficult to establish. He showed that preventive measures based on the theory had been a failure. Finally, he believed that the recurrence of symptoms of the disease for successive springs, in patients who abstained absolutely from the use of corn, militated against the theory. [Illustration: 145. Pellagrous eruption on the hand. After Watson.] On the other hand, Sambon believed that the periodicity of the symptoms, peculiarities of distribution and seasonal incidence, and analogies of the symptoms to those of other parasitic diseases indicated that pellagra was of protozoal origin, and that it was insect-borne.