The Spanish flu outbreak of 1918–1919 is considered the most devastating epidemic in history. It killed an estimated 550,000 people in the United States and 30 million people worldwide, according to the Department of Health & Human Services.

Though it did not originate in Spain, that’s where it initially set off the most alarm when, by some estimates, 8 million people died in a single month, May 1918.

In this country, the pandemic came in three waves, all associated with World War I. The first wave appeared in the spring of 1918 in military camps throughout the country. The second hit in the fall, beginning in Boston, as military supplies moved through the bustling port. That October alone, nearly 200,000 Americans died from the flu.

The end of the war brought the third wave as soldiers returned from overseas and as large groups of Americans gathered in cities and towns to celebrate Armistice Day. In all, more people died of the Spanish flu than from the war.

Anecdotes gathered by Stanford University underscore how rapidly the flu could kill: people were stricken on the street and died; workers suddenly got sick on the job and died within hours; four women played bridge one night and then returned to their homes; three were dead before dawn.

Most people who contracted the virus recovered in a few days, but about 20 percent succumbed to pneumonia. The majority of the dead were between 20 and 40 years old.

It wasn’t until the fall of 1918 that some scientists began to argue that a virus was to blame, but, as HHS reports, “virology was still very much in its infancy at this time.”

Hospitals were hard-pressed to deal with the pandemic. Many physicians were themselves serving in the war and many others contracted the disease. Meanwhile, hospitals and their staffs already were struggling to care for all the wounded soldiers who were returning home.

Philadelphia set up emergency hospitals in parish houses and state armories, according to 100 Faces of Health Care, published in 1999 by the American Hospital Association. At one point at San Francisco Hospital, designated as the isolation hospital for flu patients, 78 percent of its nurses were struck by the illness. The city also required every “person appearing on the public streets, in any public place” to wear a mask.

Experts say the pandemic did not immediately spur significant efforts to improve public health and avert future major outbreaks. Despite its terrible toll, people were eager to move on once the worst was over. Some historical accounts even refer to it as “the forgotten epidemic.”