Epidemiology.— An important epidemiological feature of pellagra, in addition to those already mentioned, is the marked fluctuation of its prevalence from year to year. At times there may be long periods of quiescence, followed by years of considerable activity during which the disease may be looked upon as a new invasion. Pellagra is in no way contagious. The sound may associate with the sick and remain healthy. Doctors, nurses, and attendants on pellagrins are not known to contract the disease. Pellagrous wet-nurses do not infect their charges, and attempts to transmit the disease by inoculation have failed.
Associated diseases such as ankylostomiasis, bilharziasis, tuberculosis, sprue, dysentery, and syphilis play a very important part in favouring the development of pellagra, in accelerating its course, in modifying and aggravating its symptoms, and in determining its mode of termination.'
The virus.— Pellagra has been ascribed to the most varied causes, such as insolation, poverty, insanitary dwellings, syphilis, irritant oils, bad water, alcohol, garlic, onions, maize. Some have regarded it as a modified or degenerate form of leprosy, others as "sunstroke of the skin," and D'Oleggio, in 1784, proposed that it should be called " vernal insolation." " Sun disease " was an old popular name, and certainly the skin manifestations of pellagra are influenced by the action of the direct rays of the sun. This was proved experimentally, first by Gherardini, who varied the limits of the eruption by systemati- cally displacing parts of the clothing ; and later by Hameau, who obtained differently shaped patches of erythema by means of gloves fenestrated in different ways. In smallpox and also in other exanthemata we notice a decided influence of light, more particularly of the actinic rays, on the production of their skin eruptions. Although light may influence the eruption in pellagra, this is no adequate reason for concluding that insolation is the cause of the disease, any more than that it is the cause of smallpox. In support of the sunlight hypothesis, certain experi-
