The global flu outbreak of 1918 killed 50 million people worldwide, ranking as one of the deadliest epidemics in history.
For decades, scientists have debated where in the world the pandemic started, variously pinpointing its origins in France, China, the American Midwest, and beyond. Without a clear location, scientists have lacked a complete picture of the conditions that bred the disease and factors that might lead to similar outbreaks in the future.
The deadly "Spanish flu" claimed more lives than World War I, which ended the same year the pandemic struck. Now, new research is placing the flu's emergence in a forgotten episode of World War I: the shipment of Chinese laborers across Canada in sealed train cars.
Historian Mark Humphries of Canada's Memorial University of Newfoundland says that newly unearthed records confirm that one of the side stories of the war—the mobilization of 96,000 Chinese laborers to work behind the British and French lines on World War I's Western Front—may have been the source of the pandemic.
Writing in the January issue of the journal War in History, Humphries acknowledges that his hypothesis awaits confirmation by viral samples from flu victims. Such evidence would tie the disease's origin to one location.
But some other historians already find his argument convincing.
"This is about as close to a smoking gun as a historian is going to get," says historian James Higgins, who lectures at Lehigh University in Bethlehem, Pennsylvania, and who has researched the 1918 spread of the pandemic in the United States. "These records answer a lot of questions about the pandemic."
Last of the Great Plagues
The 1918 flu pandemic struck in three waves across the globe, starting in the spring of that year, and is tied to a strain of H1N1 influenza ancestral to ones still virulent today.
The outbreak killed even the young and healthy, turning their strong immune systems against them in a way that's unusual for flu. Adding to the catastrophic loss of lives during World War I, the epidemic may have played a role in ending the war.
"The 1918 flu was the last of the great plagues that struck humanity, and it followed in the tracks of a global conflict," says Humphries.
Even as the pandemic's origins have remained a mystery, the Chinese laborers have previously been suggested as a source of the disease.
Historian Christopher Langford has shown that China suffered a lower mortality rate from the Spanish flu than other nations did, suggesting some immunity was at large in the population because of earlier exposure to the virus.
In the new report, Humphries finds archival evidence that a respiratory illness that struck northern China in November 1917 was identified a year later by Chinese health officials as identical to the Spanish flu.
(Related: Here's what coronavirus does to the body.)
He also found medical records indicating that more than 3,000 of the 25,000 Chinese Labor Corps workers who were transported across Canada en route to Europe starting in 1917 ended up in medical quarantine, many with flu-like symptoms.
The Spanish flu reached its height in autumn 1918 but raged until 1920, initially gaining its nickname from wartime censorship rules that allowed for reporting on the disease's ravages in neutral Spain.
Physicians began debating the origin of the pandemic almost as soon as it appeared, Higgins says, with historians soon joining them.
France's wartime trenches, ridden with filth, disease, and death, were originally seen as the flu's breeding ground. The flu's tendency to strike young adults was explained as the disease targeting itself to young soldiers in trenches. The theory also purported to explain how the illness spread from Europe to cities such as Boston and Philadelphia by pointing a finger at returning troop ships.
A decade after the war, Kansas was identified as another possible breeding ground, due to reports of an influenza outbreak there that spread to a nearby Army camp in March 1918, killing 48 doughboys.
But in his study, Humphries reports that an outbreak of respiratory infections, which at the time were dubbed an endemic "winter sickness" by local health officials, were causing dozens of deaths a day in villages along China's Great Wall. The illness spread 300 miles (500 kilometers) in six weeks' time in late 1917.
At first thought to be pneumonic plague, the disease killed at a far lower rate than is typical for that disease.
Humphries discovered that a British legation official in China wrote that the disease was actually influenza, in a 1918 report. Humphries made the findings in searches of Canadian and British historical archives that contain the wartime records of the Chinese Labor Corps and the British legation in Beijing.
At the time of the outbreak, British and French officials were forming the Chinese Labor Corps, which eventually shipped some 94,000 laborers from northern China to southern England and France during the war.
"The idea was to free up soldiers to head to the front at a time when they were desperate for manpower," Humphries says.
Shipping the laborers around Africa was too time-consuming and tied up too much shipping, so British officials turned to shipping the laborers to Vancouver on the Canadian West Coast and sending them by train to Halifax on the East Coast, from which they could be sent to Europe.
So desperate was the need for labor that on March 2, 1918, a ship loaded with 1,899 Chinese Labor Corps men left the Chinese port of Wehaiwei for Vancouver despite "plague" stopping the recruiting for workers there.
In reaction to anti-Chinese feelings rife in western Canada at the time, the trains that carried the workers from Vancouver were sealed, Humphries says. Special Railway Service Guards watched the laborers, who were kept in camps surrounded by barbed wire. Newspapers were banned from reporting on their movement.
Roughly 3,000 of the workers ended up in medical quarantine, their illnesses often blamed on their "lazy" natures by Canadian doctors, Humphries said: "They had very stereotypical, racist views of the Chinese."
Doctors treated sore throats with castor oil and sent the Chinese back to their camps.
The Chinese laborers arrived in southern England by January 1918 and were sent to France, where the Chinese Hospital at Noyelles-sur-Mer recorded hundreds of their deaths from respiratory illness.
Historians have suggested that the Spanish influenza mutated and became most deadly in spring 1918, spreading from Europe to ports as far apart as Boston and Freetown, Sierra Leone.
By the height of the global pandemic that autumn, however, no more such cases were reported among the Chinese laborers in Europe.
Humphries concedes that a final answer to the mystery of the Spanish flu's origins is still a ways off.
"What we really need is a sample of the virus preserved in a burial for the medical experts to uncover," Humphries says. "That would have the best chances of settling the debate."
For the last decade, experts such as Jeffery Taubenberger, of the National Institute of Allergy and Infectious Diseases, have sought burial samples across continents, seeking to find preserved samples of the virus in victims of the outbreak.
Taubenberger led a team in 2011 that looked at flu virus samples taken from autopsies of 32 victims of the 1918 outbreak.
The earliest sample found so far was from a U.S. soldier who died on May 11, 1918, at Camp Dodge, Iowa, but the team is looking for earlier cases.
A broad number of samples from flu victims before and after the pandemic might finally narrow down its origins. Essentially, scientists would need a genetically identified sample of the influenza's H1N1 virus taken from a victim who died before the first widespread outbreak of the pandemic in spring 1918 to point to a time and place as the likely origin point of the pandemic.
One from China in 1917, for example, would fill the bill.
"I'm not sure if this question can ever be fully answered," Taubenberger cautions, noting that even the origin of a smaller flu pandemic in 2009 still eludes certainty.
Ultimately, "these kinds of [historical] analyses cannot definitively reveal the origins and patterns of spread of emerging pathogens, especially at the early stages of the outbreak," Taubenberger said, of the new historical report.
In the end, however, knowing the origin of the disease might provide information that could help stop a future pandemic, making the search worthwhile.
"I would say that the takeaway message of all of this is to keep your eye on China" as a source of emerging diseases, Higgins says. He points to concerns about avian flu and the SARS virus, both arising from Asia in the last decade.
The SARS outbreak claimed perhaps 775 lives in 2003, and avian flu A (H5N1) has killed 384 people since 2003, according to the World Health Organization, which is carefully watching for signs of an outbreak of the diseases.
"We have seen a lot of emerging diseases travel around the world in recent decades," Higgins says.
History has a way of repeating, he says, and research into the origins of the 1918 flu could help prevent a scourge like that from happening again.
Follow Dan Vergano on Twitter.