Even in wartime, illness and noncombat injuries “have claimed far more soldiers than gunshot wounds,” Dr. Woodson said. “That forced the military to created a robust public health system. You have to take care of the troops, or you have no one left to do the fighting.” In the Army’s dawning days, George Washington ordered all his soldiers to be inoculated against smallpox.

But health concerns have always competed with other priorities, Dr. Woodson said, and there is often debate over whether they should curtail operations, as they have in suspending the major exercise in South Korea.

“A lot of time, money and people are put into these exercises, and there are always people who want them to go forward,” he said. “But I think the leadership is being properly informed about the larger consequences.”

In many ways the military is better positioned than the civilian population to respond to a global outbreak. The armed forces are a young, healthy population that has universal health care. Bases with gates can easily limit access. Commanders have far more authority than civilian leaders to impose quarantines and vaccinations, close facilities and order troops to stay away from public gatherings.

“It’s the ideal public health environment, but it’s a double-edged sword,” said Carol R. Byerly, a former research historian for the Office of the Army Surgeon General. “You have great resources, great monitoring, but the needs of the military can also make the military more vulnerable.”

She noted that during World War I, the deadly 1918 flu pandemic was made worse by the mobilization of tens of thousands of young men who were concentrated in cramped Army training barracks.

The Army’s expert on communicable disease at the time, Dr. Victor Vaughan, visited Camp Devens in Massachusetts and was shocked by what he saw: “Hundreds of stalwart young men in the uniform of their country coming into wards of the hospital in groups of ten or more. They are placed on the cots until every bed is full, yet others crowd in. Their faces soon wear a bluish cast; a distressing cough brings up the bloodstained sputum. In the morning, the dead bodies are stacked about the morgue like cord wood.”