This paper will examine the growth of early air bacteriology studies within the United States and their relevance to both public health and later military projects through an examination of research conducted by sanitary engineer William Firth Wells. Research conducted by Wells at the Harvard School of Public Health from 1930-1937, and at the University of Pennsylvania Medical School's Laboratory for the Study of Airborne Infection from 1937-1944, produced experimental evidence for the existence of droplet nuclei as a method for airborne contagion. Wells's research challenged earlier findings by Carl Flügge on droplet infection (1897), and more importantly, on Charles V. Chapin's concept of contact infection (“The Sources and Modes of Infection,” 1910). During World War II, research in aerobiology by medical and public health practitioners resulted in both civilian and military research programs including work on airborne biological weapons. The postwar success enjoyed by military researchers and public health researchers in understanding the dynamics of airborne disease was due in measure to Wells's earlier research.