As World War I raged in Europe, an even deadlier killer was on the loose — influenza. During 1918, up to 50 million people died during the worst flu pandemic the world has ever seen.

About 675,000 perished in the United States alone — far more than the number of Americans who died in World War I. You can experience those terrifying days through "The Deadly Virus," an online exhibition from the National Archives and Records Administration.

There are photos of masked policemen patrolling city streets and letter carriers doing likewise to avoid contagion. A nurse — one of more than 700 recruited to stem the tide of the disease — wears a mask, too. So does a streetcar conductor in Seattle who refused to let passengers board a trolley without a mask.

The masks were unsettling enough, but the disease's sheer sweep was even more terrifying. Other records show hospital admissions, and document the ways in which counties, military branches and the Bureau of Indian Affairs responded to the outbreak.

A letter of condolence drives home the very real possibility of death by flu. "This disease which has taken thousands upon thousands throughout the country was no worse here than elsewhere," writes an obviously frustrated Indian Agency superintendent in Yakima, Wash.

Ultimately, an estimated 28 percent of all Americans were infected with the flu. "The Deadly Virus" is a reminder of the human costs and organizational challenges of the world's worst pandemic.