We are accustomed to think of measles as a disease which, like meningitis, is disseminated throughout the civilized world, and which although constantly with us causes only sporadic cases. True epidemics of measles do occur, even when there is no gross change in the constitution of the population. We have discussed examples of this particularly in London. Flare-ups of measles prevalence are in fact so much the rule that in certain localities health officers anticipate a measles epidemic about once in two years. Furthermore measles has been known more than once to occur in extensive epidemic form, attacking large proportions of the population invaded. We know that there is an immunity to measles which is nearly absolute in those who have once acquired the disease. The epidemics have, therefore, occurred exclusively in those localities where the proportion of immune individuals was relatively small. According to Noah Webster in 1772 measles appeared in all parts of America and was accompanied by an unusually high mortality. In Charleston 800 or 900 children died of the disease. The following year measles “finished its course and was followed by a disorder of the throat.” In 1781 measles disappeared from the Faroe Islands, and for the following sixty-five years there was not a case of this disease anywhere on any of the seventeen islands constituting the inhabited parts of this group. When the disease was finally re-introduced into these islands, it spread throughout the population, attacking practically every individual in a relatively short interval of time, showing a much higher attack rate than did influenza in 1918. There was this difference, that the only individuals who did not acquire measles on its re-introduction into the islands were those who had had it sixty-five years before. Panum did not find an authentic case of recurrence in the same individual. There was not a single instance of second attack of measles, although the shortest possible interval between the previous attack and the subsequent exposure was sixty-five years.