Zinsser taught as an exchange professor and worked with the American Red Cross in France, Russia, Serbia and China, and was noted for his work in typhus and immunology. He became a lieutenant colonel in the US Army and served overseas during World War I. He was awarded the Distinguished Service Medal, the citation for which read as follows "For exceptionally meritorious and distinguished services-- while acting as Sanitation Inspector of the Second Army he organized, perfected and administered with extraordinary and exceptional success a plan of military sanitation and epidemic-disease control." Zinsser also received another military citation for taking exceptional risks to minister to wounded soldiers while under direct enemy fire. He was also awarded the Order of St. Sava of Serbia and the Legion of Honour in France.