Final answer:
The disease that caused 1/3 of total military deaths during World War 1 was the influenza epidemic. The conditions of World War I greatly contributed to the spread of this disease.
Step-by-step explanation:
The disease that caused 1/3 of total military deaths during World War 1 was the influenza epidemic.
The influenza epidemic immediately after the First World War killed at least 21 million people, and probably many more, compared with the 8 to 10 million soldiers killed in action. The conditions of World War I greatly contributed to the spread of this disease, including crowding, poor sanitation, and rapid mobilization of large numbers of personnel and animals. The influenza pandemic of 1918-1919 is commonly known as the Spanish flu and is thought to have originated in the United States.