It started on a small military base in central Kansas, when one soldier came down with a fever. Within a few hours, about 100 soldiers had reported to the Fort Riley infirmary with the same complaint.
By 1919, one year later, the so-called Spanish flu had spread around the world, killing an estimated 50 million people, with more than 500,000 dead in the U.S. (That included 195,000 just in the single month of October 1918.) The disease took more lives than the black plague, and more than all the wars of the 20th century combined.
In especially virulent cases, many of the afflicted died just hours after feeling their first symptoms. Death estimates vary because people often died and were buried before they could be named or counted.
Health experts are concerned that the Spanish flu that ravaged the world has many similarities to the avian flu now found throughout Southeast Asia.
The 1918 flu strain itself originated as an avian flu that mutated into a form that could jump between humans. And the 1918 strain, according to research published this week in the scientific journals Nature and Science, has several of the same genetic mutations as the current avian flu strain.
Could a deadly pandemic happen again? Some doctors say yes.
Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn., says there are some parallels between the avian flu and the Spanish flu.
"It was a new influenza strain at the time," Schaffner said of the Spanish flu. "The population of people on the whole planet had no experience with it."
The longer a flu strain has been around, the more time people have had to build up resistance to it.
Another striking parallel between the two diseases is the victims. While most flu viruses strike the very young and very old, the Spanish flu affected many between the ages of 20 and 35. The avian flu has followed a similar pattern.
"It particularly had a high fatality rate among young, healthy people," Schaffner said of the 1918 pandemic.
The flu that struck Fort Riley killed 48 soldiers in the spring of 1918. It might have been contained there but for one thing: World War I.
Soldiers gathering in military bases were shipped out to Europe that year, bringing with them the deadly virus. Though the Spanish flu was spread aboard ships and in the trenches and military camps of Europe and America, Schaffner notes the avian flu might be dispersed even more quickly and easily today.
"It would spread much more rapidly on airplanes," Schaffner notes. "The potential for rapid spread is much greater today."
With the Spanish flu spreading uncontrolled in cities and rural areas alike, a breakdown in the social fabric of many communities came quickly.
Orphans wandered the streets while bodies piled up in makeshift morgues. Coffins were in short supply so open carts picked up the dead from front porches. Many bodies were hurriedly buried in mass graves dug by steam shovels.
Panicked communities across the country enacted a variety of laws to address the invisible enemy. Public gatherings of all kinds – even in saloons – were outlawed. Handshakes became illegal. Funerals were not allowed to last more than 15 minutes. Face masks of thin gauze (though worthless against the virus) were required by law in some areas.