How much of that was down to the disease, compared to the conditions that "young, healthy" people would have been subjected to during a world war?
"Why did so many young adults die? As it happens, young adults have the strongest immune systems, which attacked the virus with every weapon possible—including chemicals called cytokines and other microbe-fighting toxins—and the battlefield was the lung. These “cytokine storms” further damaged the patient’s own tissue."
The more robust immune response was a large cause of the increased mortality.
I think the war definitely helped spread it as living conditions were rough and there was plenty of migration.