Period from 1893–1918 47 Increase in 1900–1901 49 Period from 1901–1915 51 Influenza in 1915–1916 55 Influenza between 1916 and 1918 58 The pandemic of 1918 59 Date of first increased prevalence in various localities 65 Influenza in China 80 Autumn spread in the United States 81 Recrudescences 87 Recurrences in the winter of 1919–1920 89 The winter of 1920 90 Incubation period 95 Predisposing causes 96 Periodicity 97 Virulence enhancement 108 Meteorologic conditions 113 Secondary invaders 114 Origin of the 1918 pandemic 116 SECTION III. AN INVESTIGATION OF INFLUENZA IN BOSTON (WINTER OF 1920.) 127 Diagnostic standards for the 1918 epidemic 134 Standards for 1920 134 Morbidity 137 Relation of sex to morbidity 143 Relation of sex to severity 145 Morbidity by age 145 Relationship of occupation to morbidity 150 Effect of race stock 155 Mortality 156 Mortality by sex 165 Relationship to age 167 Relationship of occupation 170 Density of population 170 Race stock and mortality 173 SECTION IV. AN INTENSIVE STUDY OF THE SPREAD OF INFLUENZA IN SMALL GROUPS OF CLOSELY ASSOCIATED INDIVIDUALS 175 Effect of overcrowding 179 Domestic cleanliness 187 Economic status 189