James Simmons began his career in the US Army as a laboratory officer and his assignments progressed into tropical medicine research. His interests and work evolved into preventive medicine (PM, as the Army termed public health), and he took both a PhD and a Doctorate in Public Health. As the Army's leading PM officer he was appointed head of PM in 1940 and guided the Army's PM effort through World War II. His responsibility ran from gas masks through healthy nutrition and occupational health to an enormous variety of diseases; by the war's end, the breadth and importance of PM was reflected in the Preventive Medicine Division, having fully one-sixth of all military personnel at the Surgeon General's Office. Simmons used his strong professional credentials to tap into civilian medicine for expertise the Army lacked and he established organizations that survive to this day. After retirement, he sought to expand the field of public health and raise another generation of public health physicians.