Background. Surveillance for respiratory diseases in domestic National Army and National Guard training camps began after the United States’ entry into World War I, 17 months before the “Spanish influenza” pandemic appeared.
Methods. Morbidity, mortality, and case-fatality data from 605 625 admissions and 18 258 deaths recorded for 7 diagnostic categories of respiratory diseases, including influenza and pneumonia, were examined over prepandemic and pandemic periods.
Results. High pandemic influenza mortality was primarily due to increased incidence of, but not increased severity of, secondary bacterial pneumonias.
Conclusions. Two prepandemic incidence peaks of probable influenza, in December 1917–January 1918 and in March–April 1918, differed markedly from the September–October 1918 pandemic onset peak in their clinical-epidemiologic features, and they may have been caused by seasonal or endemic viruses. Nevertheless, rising proportions of very low incidence postinfluenza bronchopneumonia (diagnosed at the time as influenza and bronchopneumonia) in early 1918 could have reflected circulation of the pandemic virus 5 months before it emerged in pandemic form. In this study, we discuss the possibility of detecting pandemic viruses before they emerge, by surveillance of special populations.