TORONTO - The Allied Forces were gaining hard-earned ground in the late summer of 1918, carving a path that would shortly lead to an armistice for the First World War.

As those armies battled over the blood-soaked fields of Europe 90 years ago, another enemy was on the move. This adversary didn't choose sides. It didn't restrict itself to Europe. It didn't spare civilians.

By the time it was through rampaging around the globe, this assailant - the Spanish flu - had killed between 50 million and 100 million people, several times more than had lost their lives in the soon-to-be-concluded War to End All Wars.

Yet despite the scope of the death and illness, the fear and social disruption, amazingly - inexplicably - the history books made little mention of what is now viewed as the deadliest outbreak of infectious disease in recorded history.

"If I hadn't researched and written a book myself, I'd want to go look it up in the Encyclopedia Britannica to find out whether it really happened or not," historian Alfred Crosby said in a recent interview from his home on Nantucket Island.

Crosby's 1976 account of the event - "America's Forgotten Pandemic: The Influenza of 1918" - sank the first spade in what has turned into a historical and scientific excavation of the Spanish flu.

"For me, that's the most mysterious thing about it," Crosby says. "The vagaries of the virus, we'll understand them eventually. And we'll understand how flu epidemics work."

"But we're never going to understand: How the hell did we have something that killed millions and millions of people and then we said 'Oh, well' and went on to the World Series or something?"

"It's impossible. And yet it's true."

Most of us would have had a relative - an aunt, a grandfather, a great-grandmother - who was sick with the Spanish flu. If we'd asked or if they'd offered, they could have told stories of a taxing time, when gymnasiums morphed into crowded makeshift hospitals, when undertakers ran out of coffins, when the Stanley Cup playoffs were shelved - the only time that happened until the NHL lockout of 2004-05.

In fact, there are still some among us who can recall those dreadful months in the fall of 1918 and the spring of 1919. Rev. Francis Stevens of Coquitlam, B.C., is one.

Now nearly 102 and a retired United Church minister, Stevens remembers vividly that it felt like the world was spinning out of control when the Spanish flu coursed through his Vancouver neighbourhood.

His entire family was sick. Stevens, then 12, caught the flu first and recovered, only to find school closed and chums either bedridden or forbidden by frightened parents from playing with others who might infect them.

"You were kept in. Kept in at home, kept out at school," Stevens recalls. "Your school and your home were your two places of security. And both were collapsing."

Recounting the details now, the story seems like bad science fiction - 50 million to 100 million dead globally, 30,000 to 50,000 dead in Canada. If a similarly virulent outbreak occurred now, between 186 million and 372 million people around the world would die, and between 112,000 and 186,000 Canadians would succumb.

Today such a catastrophe would be non-stop news. Consider the SARS outbreak of 2003: the disease infected just over 8,000 people and killed fewer than 800, but rivalled the start of the war in Iraq for top-of-the-newscast status.

Not so in 1918. Accounts of the war in Europe crowded news about the flu bug out of the front pages of newspapers. "Usually it was on Page 12," Crosby notes.

Heather MacDougall, a historian at the University of Waterloo, has studied Canadian newspaper coverage of the time.

"The press was heavily self-censored because the war was still on. And when you look at the news stories, the rhetoric of the stories is that this is just another battle that we have to fight. Except it's against disease, not against the Germans."

"And given that we are now finally winning the war against the Germans, we will win this war against disease."

It has been suggested the curious under-reporting of the event in countries that were combatants in the First World War contributed to the evolution of the outbreak's moniker, which implies the virus arose in Spain. (Influenza viruses are named after the place where they are first found.)

The thinking is that because Spain was a non-combatant, its newspapers were more openly reporting on the alarming new twist on an old illness.

Influenza viruses circulate all the time, sending victims to their beds with bone-aching fatigue. They can even kill. In Canada, between 4,000 and 8,000 people a year - often the elderly - die from influenza or the pneumonia that can follow. But occasionally a new influenza virus for which humans have little or no immunity will emerge from nature, causing a global outbreak of disease known as a pandemic.

There appears to be no cycle or pattern involved. Nine years separated two pandemics in the mid-1800s, but it's now been 40 years since the most recent pandemic, the 1968-69 Hong Kong flu.

American historian John Barry, author of the 2004 book "The Great Influenza," believes the virus responsible for the 1918 pandemic emerged in the spring in the U.S. heartland, probably Kansas. Unusually virulent outbreaks of influenza were reported in some military camps there. Later, Barry and others believe, troop ships took the virus to the battlefields of Europe, where massive numbers of soldiers on both sides fell ill.

The virus went quiet in the northern hemisphere's summer, but reports started to crop up of renewed sickness in Europe in late August. In September, the illness erupted at Camp Devens, a military base near Boston.

In Canada, the first report of an outbreak among civilians occurred at Victoriaville College in Quebec on Sept. 8. School officials elected to send home students who were well enough to travel, undoubtedly sending flu along with some of them.

It was during this second wave of the pandemic that the new influenza virus turned preternaturally deadly.

It's thought that roughly 2.5 per cent of people who caught the flu died from it - an extraordinarily high rate for influenza. But the toll was substantially worse among certain groups - pregnant women, aboriginals and young, previously healthy adults.

When influenza kills, it normally claims the very young or the very old. But for reasons scientists are still trying to puzzle out, young adults were the hardest hit age group.

In remote communities in the U.S. and Canadian North, the devastation reached apocalyptic levels.

According to Eileen Pettigrew's "The Silent Enemy: Canada and the Deadly Flu of 1918," only 70 people of 220 survived in the Labrador town of Hebron. At Okak, also on the Labrador coast, 207 of 266 people died. The survivors, who later abandoned the site, fought to keep starving sled dogs from devouring corpses.

In the Western Arctic and northern British Columbia, First Nations and Inuit communities were also ravaged.

"It struck with the same kind of ferociousness or ferocity in aboriginal communities as it did in non-aboriginal communities," says Mary-Ellen Kelm, a social historian who has studied the Spanish flu's impact on native communities in British Columbia.

"But the death rates (in aboriginal communities) were ... I think about seven times the rates for non-aboriginal for British Columbia as a whole."

Kelm, a professor at Simon Fraser University in Burnaby, says high rates of tuberculosis may have made aboriginal people more vulnerable to the viral assault. And there is evidence that some of the young children who died didn't succumb to flu, but to neglect. There were no adults to care for them.

Everywhere communities struggled to cope with the sick and the dead.

"The undertaking parlours couldn't handle the bodies as people died," says Louise Brooks, 99, of Vancouver, who battled the flu herself. "I have this vague memory that they were having to use school auditoriums and places like that to store bodies temporarily."

With hospitals overwhelmed, most people struggled through the illness at home. Most pulled through, even without the help of antiviral drugs or vaccine. (Vaccines were made, but against the wrong germ. Science hadn't yet discovered viruses and doctors thought the bacterium Bacillus influenzae was to blame.)

The lucky folks had neighbours or relatives who stepped in to help.

There were cases of heroism. Pettigrew's book noted the hamlet of Beaubier, in southeastern Saskatchewan, was named after Eleanor Beaubier, a teacher who tirelessly nursed the sick before perishing herself.

Anna Shillinglaw, 97, remembers neighbours who helped her family pull through when both parents and all seven children were struck down with flu. A boy, Henry Kindopp, would come to tend to her father's animals on their homestead near Bitter Lake, Sask.

"And his mother cooked soup and they put it in a big lard pail and he brought it to the house for us so we had something to eat," recalls Shillinglaw, who lives in Langley, B.C.

As Kelm notes, in other places the lack of such aid contributed to the death toll. Ottawa's mayor, Harold Fisher, issued an appeal for the well to help the sick.

"I want to make it absolutely clear that people are dying in our midst because they are not provided with proper care," Pettigrew's book quotes Fisher as saying.

"They are not dying because we don't know about them. We know where they are, but we have nobody to send. Knitting socks for soldiers is very useful work but we are now asking the women of Ottawa to get in the trenches themselves."

Today, scientists are still trying to figure out why the virus was so deadly. Back then, doctors were trying and failing to cope with a flu bug that caused regular influenza in some people and a rapidly progressing and devastating disease in others.

"There was terror," says Barry, who notes doctors, who felt medicine was on the verge of conquering infectious diseases because of scientific advances, were helpless in the face of the onslaught.

"It kicked them right in the face and destroyed their confidence," he notes. "And of course society itself was just overwhelmed."

"People could die in less than 24 hours after the first symptoms. And they could die with horrific symptoms. People could bleed not only from their nose and mouth, but from their eyes and ears. People were turning so dark blue from lack of oxygen that physicians were saying it was difficult to tell whites from blacks."

The virus swept round the world like a firestorm in the fall of 1918. After a brief respite, a third wave hit in the spring of 1919; in some places the third wave occurred in 1920.

And then the virus seems to have weakened and flu seasons resumed their normal pattern.

The bug, an H1N1 flu virus, continued to circulate for decades. In fact, descendants of the virus still circulate, though they are now the milder of two influenza A viruses that cause disease each winter.