Differentiation of the smallpox eruptions similar in type has become more certain and rapid through the work of Ricketts.
Ricketts' thesis is that the focal rash of smallpox has a character istic distribution governed by two complementary factors, viz.: exposure to irritation and protection against irritation, which determine a relative increase and decrease of the incidence of the rash on any part of the skin surface. These factors are indepen dent of the course of the disease and of the evolutionary changes which constitute the features of the rash. They are also indepen dent of modification of these changes resulting from variation in immunity possessed by the patient or of variation in the toxic power of the infecting virus. They have the further advantage of being objective and therefore less liable to confusion in inter pretation than any others. They begin to be established with the beginning of the outcrop of the rash, are fully established when eruption is completed and remain unchanged as long as the rash lasts.
Blaxall, Cleland and Ferguson, Green and Gordon in England, and Leake and Force in America, have experimented on lower animals with material from the mild disease and also with mate rial from severe cases of acknowledged smallpox, in order to determine what differences, if any, exist between the two condi tions and in their immunological relations to each other and to vaccinia. The results of these investigations have been sum tnarised by Ledingham, and confirm the opinion that the mild disease as it exists in England is smallpox of which the virus has lost a degree of its toxicity for man but retains its important properties for other animals. The distinction between the two
viruses is one of lethal power only and the factors which govern this property are unknown. Over five years' experience in England of a widespread infection by the mild type of smallpox has afforded very little, if any, evidence of a tendency to increase in virulence. But in the absence of knowledge of the cause of variation in lethal power or of the natural conditions which may favour its occurrence, it is unsafe to assume that a low degree of virulence is a permanent feature of this type of smallpox and on that assumption to relax the stringency of administrative pre cautions against the disease.
Smallpox cases as reflected in the U.S. Public Health Reports show a seasonal fluctuation, with an upward tendency in the U.S. during 1927-28. In analogous centres of civilization in Europe and the United States, the former shows a greater control of the disease due to the fact that state officials in Europe have an organized system of inspection and the machinery for compulsory carrying out of regulations, not yet established in the United States. Vaccination of children and revaccination of adults is stressed by French and American authorities as essential to the restraint of the disease.