DURING the past 40 years the ecological approach to disease has become a basic concept of epidemiology. Among all diseases measles has stood as the classic example of successful parasitism. This self-limiting infection of short duration, moderate severity, and low fatality has maintained a remarkably stable biological balance over the centuries. Those epidemiologists, and there are many, who tend to revere the biological balance have long argued that the ecological equilibrium of measles is solidly based, that it cannot readily be disrupted and that therefore we must learn to live with this parasite rather than hope to eradicate it. This speaker, not so long ago, was counted among this group and waxed eloquent on this subject in print.' Happily, this era is ending. New and potent tools that promise effective control of measles are at hand. If properly developed and wisely used, it should be possible to disrupt the biological balance of measles. Its eradication from large continental land masses such as North America and many other parts of the world can be anticipated soon. The importance of any disease as a public health problem must be gauged from many angles. For example, using mortality as a criterion heart disease becomes most important. Short-term morbidity makes the common cold rank high. For chronic disability arthritis and mental disease dominate. For public interest and parental concern, in spite of relatively low incidence, nothing has equaled poliomyelitis. According to these criteria, the importance of measles cannot be compared with any of the diseases mentioned so far, but it should still be classed as an important health problem on two main counts. First, any parent who has seen his small child suffer even for a few days with persistent fever of 105°, with hacking cough and delirium wants to see this prevented, if it can be done safely. Second, at last there is promise that something can be accomplished by organized health action. As a contribution to this symposium, we of the Communicable Disease Center have brought together some of the basic descriptive statistics concerning measles in the United States. We hope this may serve as a simple frame of reference broadly defining our problem. Figure 1 presents annual morbidity and mortality for the expanding reporting areas from 1912 to 1959. Note the stability of the morbidity rate and the steady downward trend in the mortality rate. Also, there is the somewhat ominous suggestion of a cessation of this downward trend since 1955 similar to the leveling off of the infant death rates during the past six years. The morbidity figures testify to the stability of the biological balance of measles during the period. The decline in mortality demonstrates the degree to which we have adapted to this balance and have learned to live with this parasite. Figure 2 presents the familiar curves of cumulative frequency of a history of measles by age. Two large studies published by Collins in 19292 and 19423 are compared with a recent survey conducted by Epidemic Intelligence Service