The house-to-house census was carried out with the co-operation and assistance of the Federated Jewish Charities in Boston and individual members of the social service departments of the Massachusetts General Hospital and the Peter Bent Brigham Hospital. THE AUTHOR. Department of Preventive Medicine and Hygiene, Harvard Medical School, Boston, _September 17, 1920_. This monograph is one of a series of studies carried out under the supervision of the Influenza Commission of the Metropolitan Life Insurance Company. The Influenza Commission is investigating the etiology and prevention of the acute respiratory diseases, and the work is being carried on in Boston, New York City, Chicago, and Washington. The members of the Commission are Dr. M. J. Rosenau, Chairman, Dr. G. W. McCoy, Dr. L. K. Frankel, Dr. A. S. Knight, Dr. E. O. Jordan, Dr. W. H. Frost and Dr. W. H. Park, Secretary. INFLUENZA. AN EPIDEMIOLOGIC STUDY.[1] BY WARREN T. VAUGHAN, M. D. (Received for publication April 6th, 1921.) CONTENTS SECTION I. PAGE GENERAL EPIDEMIOLOGIC CONSIDERATIONS 1 Historical 2 Clinical and epidemiologic identification 13 General characteristics of early epidemic outbreaks 14 Symptoms in former epidemics 19 Manner of spread 20 Human intercourse 23 Crowd gatherings 26 Mass attack 27 Healthy carriers and convalescents 30 General manner of spread in individual localities 31 Primary type of epidemic 31 Secondary type of epidemic 33 Mortality curves 36 Duration of explosive outbreak 37 Morbidity curves in 1920 recurrences 41 Spread in countries and continents 42 Spread in primary waves 42 Spread in recurrences 44 SECTION II. INFLUENZA EPIDEMICS SINCE 1893 47 Occurrence since 1893 47