It’s a grim picture.

If a new human influenza virus developed and struck Japan, it could infect a quarter of the population, cause as many as 640,000 deaths and force 40 percent of the workforce to stay home, according to a Japanese government estimate.

The country’s plan for any future influenza pandemic calls for shutting down airports, closing schools and organizing mass cremations of the dead in hopes of keeping the virus off its shores or at least containing it.

U.S. Forces Japan and its service components have contingency plans for dealing with any major outbreak, said Marine Corps Master Sgt. Donald Preston, a USFJ spokesman at Yokota Air Base. U.S. and Japanese hospitals also have stockpiles of medication such as Tamiflu.

"Embassy officials and other U.S. leaders would work with the Japanese to ensure help for Americans and disseminate news," he said. "If a human outbreak did occur, it’s likely the government would ask people to remain at home, curtailing any outside activity until the outbreak is contained."

He declined to say whether USFJ would follow Japan’s move and halt military operational and Space-A passenger flights at its mainland Japan and Okinawa bases.

Flu pandemics can break out when a new virus emerges and sweeps through a population without immunity.

Dr. Eric Morgan, an Army lieutenant colonel, family practice physician and deputy commander for clinical services at the Brig. Gen. Crawford F. Sams U.S. Army Health Clinic on Camp Zama, said the belief in medical circles is that the next pandemic bug will come from Southeast Asia — a "higher-risk" area and the birthplace of severe acute respiratory syndrome, or SARS.

Morgan said the next pandemic will hit 30 percent of the worldwide population, with 1 percent resulting in fatalities. It would affect about half of all Tokyo residents, due the metropolitan area’s density, and take roughly six weeks to spread through the populace, he added.

"Normal medical response will be overwhelmed," he said. "If we can’t prevent it, we’ll at least try to slow it down."

Japan’s plan would keep open just four airports and three ports, where a strict quarantine would be aimed at preventing the virus’ entry if it emerges overseas. Those infected would be asked to stay abroad and foreigners would be restricted from entry.

In the 20th century, pandemics broke out in 1918, 1957 and 1968 — affecting up to 35 percent of the global population.

The deadliest pandemic on record, starting in 1918, killed an estimated 40 million people.

There were 650,000 deaths in the U.S. alone as it spread briskly through military ranks because of World War I. After a military transport ship arrived in France from the United States, it was discovered that 80 percent of the ship’s population died during the voyage, according to Morgan.

"Two million Americans would die if you extrapolated that out to 2007 census figures," he said.

Because of modern globalization, he added, any future pandemic is likely to spread quicker.

"There were three waves in 1918. You have to expect a bigger, more dramatic wave this time," he said.

The Associated Press contributed to this story.

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