Influenza has been one of the great mass killers in human history and its most lethal version was the Spanish flu epidemic in the fall of 1918.

At least 21 million people died worldwide, more than were killed in the fighting in the First World War. (Some historians say that large numbers of flu deaths went unreported in less developed countries. Recent research has shown that as many as 20 million people could have died in India, raising the death toll to between 40 million and 50 million.)

Soldiers returning home from the trenches at war's end didn't come back alone. They brought with them the flu virus. By the time it had run its course, 50,000 Canadians were dead. Some smaller villages in Quebec and Labrador were almost wiped out.

In the United States, 675,000 people died in the epidemic.

People today tend to think of the flu as an uncomfortable inconvenience. Then it was a serial killer that seemed to its victims to be some form of curse. Medical facilities were swamped. The killer flu struck quickly and inexplicably. Some people would go to bed healthy and never wake up.

Although the flu normally kills the very young and the very old, this epidemic was most virulent among those aged 20 to 40.

Doctors and hospitals in North America were already overtaxed by the war, with many health care workers overseas and local hospitals caring for evacuated wounded. Volunteers stepped forward to help. No one knows how many became victims themselves.

Canadian streets had an eerie look. Almost everyone who went outdoors wore a face mask. Today's scenes of masks in the street would not be out of place in the terrified Canada of the fall of 1918.

People in closed communities were most vulnerable. The flu spread rapidly through U.S. army camps filled with men who had not deployed overseas. It was reported that 500 prisoners at San Quentin Penitentiary in California were affected.

In some communities, it was a criminal offence to shake hands. Gatherings of more than six people were banned. Schools, theatres and other public buildings were closed. In the U.S., railways demanded passengers have a document certifying they were free of the flu.

American studies show that flu killed more poor people than rich, especially since many of the poor were living in crowded conditions that made transmission easy.

Back then, it would take a year for such a virus to make its way around the world. Although the virus probably originated in China, as do most flu virus variants, this flu appeared first in the British trenches on the Western Front in April 1918, then among German forces a few days later, then among French troops; it was thought the disease was caused by the horrible conditions of trench warfare.

It was called the Spanish flu because it was first officially noticed in Spain in May 1918. It went on to kill an estimated eight million people there.

The virus was tracked along international shipping lanes, from Europe to North America, then to Asia, Africa, Brazil and eventually the South Pacific.

Now in the age of international jet travel, a virus can spread in a few days. Epidemics are unpredictable in their timing but they do occur in cycles. And most scientists agree we're due.