“There is no reason for supposing that the virus of measles is controlled in any way by our calendar. In order to get anywhere in determining any law of periodicity in epidemics we must know the morbidity and mortality of the disease by days, or at least by weeks. In different parts of a large city there may be, and undoubtedly are, epidemic waves of measles on the flow or on the ebb at the same time. The best work that has been done along this line is that of Brownlee, who has figured out epidemic waves of measles, based on the weekly numbers of deaths in London between 1840 and 1912. “The figures presented by Brownlee are of great value, and his theory is fascinating and has much in its favor, not only in a study of epidemics of measles, but of the other infectious diseases of infancy and childhood, especially scarlet fever, whooping cough, and chicken-pox. In order to solve the problem of periodicity in measles we must have more exact information than we now possess. Brownlee’s figures pertain to deaths only. There are, so far as we know, nowhere in the world satisfactory statistics concerning morbidity in this disease. Deaths from measles are so largely determined by the care bestowed upon the sick and upon the extent to which secondary infection is prevented that we are inclined to hesitate about the acceptance of a death rate or number of deaths from this disease as an index to the virulence of the organism causing the disease; in other words, we are not convinced that the death rate in a given outbreak of this disease is a measure of the virulence of the organism causing it. This involves the question whether measles _per se_ is a disease of wide variation in malignancy or are the widely different death rates observed in different epidemics due to secondary infections. The streptococcus, a common invader of the body during the progress of a measles infection, is known to possess a most variable degree of malignancy. We are inclined to the opinion that if all cases of measles could be recognized before secondary infection occurs and could be cared for ideally the death rate from this disease in different epidemics would be much more uniform than is now shown and would be low. The greatest danger to life in an attack of measles lies in the fact that the virus lowers the resistance of the body cells and opens gateways to more deadly organisms, such as the streptococcus. We believe that there are demonstrated facts which support these ideas. Quite uniformly in measles there is a well marked leukopenia. As we now interpret it, this means a decrease in the number of the forces that naturally protect the body against the invasion of foreign cells. Again as we interpret it, the failure of the body cells to respond to the tuberculin test during a course of measles or soon thereafter is evidence that the resistance of the body is lowered. If our interpretation on these points be correct we fail to see how deaths from