Typhus
As the winter of 1914-1915 deepened, typhus spread across Serbia, especially in the ranks of the armies. The daily caseload at the American Red Cross Hospital surpassed 9,000 patients. Patients sometimes had to share beds. In early Spring, some patients were bundled up and put in beds in the open courtyard, the hospital thereby learning that the courtyard had a capacity of 3,000 patients. With the advent of warm weather, the epidemic finally receded.
It is no surprise that a number of staff at the hospital contracted typhus. Two of the original American nurses got it, as did Dr. Ryan. More nurses had arrived from America, and some of them caught it. Two nurses reportedly died from it, although this has proved hard to confirm. While Dr. Ryan was trying to fight it off, he was replaced as Chief Surgeon by Dr. Ernest Magruder, who had been sent from the U.S. to Serbia to study the epidemic. Ironically, soon after stepping into Ryan’s role, Magruder himself died of typhus, becoming the second American Red Cross surgeon to die of it in Serbia.
Dr. Ryan eventually made a full recovery. He continued to work in Serbia well into 1916, and went on to serve until his early death with the American Red Cross abroad. In 2018, a Serbian postage stamp commemorated his service to the country:

BTW, in the background photo on the stamp, Ryan is sporting a bow tie, and the high-foreheaded nurse standing behind his left shoulder is Mary Gladwin.