The purpose of this paper is to review certain general features of the epidemiology of meningococcus meningitis in the light of a survey of meningococcus carriage which was carried on continuously from December, 1941, to May, 1945, with a final carrier test in December, 1946. This was part of a survey including hemolytic streptococci, Corynibacterium diphtheriae and Hemophilus influenzae, which was conducted by the Commission on Pre-epidemic Survey in the First Service Command under the Board for the Investigation and Control of Influenza and Other Epidemic Diseases in the Army, Preventive Medicine Division, Office of the Surgeon General, United States Army. Of the group of diseases on which carrier studies were done, meningitis was the only one that presented an epidemic situation during the period of the study which made worthwhile a more extensive study of the relationship of findings in the carrier studies to the occurrence of meningitis during the same period, and as well to the epidemiology of meningitis in general. As it happened, the carrier study embraced the whole period of one of the cyclic increases and decreases in the occurrence of meningitis to be pointed out, and extended through four seasonal waves of the disease. Thus, the data available for analysis are unique in that carrier rates for the several types of meningococci are now known both for epidemic and interepidemic periods, as well as for epidemic and interepidemic seasons. This should not only provide a better understanding of the role of the carrier rate in the epidemiology of meningitis but at the same time should clarify certain questions concerning the mechanisms which influence the carrier rate-chiefly that of "overcrowding", long held to be a major determinant first in an increase in carrier rates and, in turn, in an increase in the disease, particularly under conditions of mobilization. Meningitis is extraordinary in the wide extent to which its infectious agent is distributed in the population and the comparative infrequency of disease in those harboring the organism. Furthermore, there is no reliable evidence that immunity from previous exposure is a determinant in restricting the occurrence of the disease, nor that population immunity, as is clearly the case with a number of virus diseases, stands in the way of repeated harborage and transmission of the organism by the same individual. Nonetheless, it has become clear that the extent of the occurrence of the disease is by no means wholly a simple function of the prevalence of meningococcus carriers. Epidemics may occur in certain 1 These studies were supported in part through the Commission on Epidemiological Survey of the Army Epidemiological Board, Office of the Surgeon General, United States Army.

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