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“British officials knew that whatever the plague was, there were Chinese labourers sick with it,” Humphries said in an interview. “And they were being shipped across Canada.”

About 50,000 Canadians would die of the contagion. Globally, an estimated three per cent of the planet’s population died between 1918 and 1920.

According to the World Health Organization, influenza is responsible for 250,000 to 500,000 global deaths each year. While most of the dead are 65 and older, however, Spanish influenza targeted young, healthy adults aged 18 to 40.

Humphries’ theory hinges on a plague that swept through parts of China in 1917. When the disease that became known as the Spanish flu appeared the following year, the symptoms were identical to those of the 1917 contagion.

“People thought it was the return of the same plague they had seen the year before,” he said. “It was investigated and it was determined in fact to be Spanish influenza. To me, that’s a smoking gun.”

Humphries’ study notes that the 1918 flu strain’s unique potency derived from a highly unusual combination of genetic characteristics that “turned the body’s immune system against itself, making it most deadly to those with particularly robust defences.”

The study follows Humphries’ acclaimed 2013 book on the Spanish flu, The Last Plague: Spanish Influenza and the Politics of Health in Canada, and details how the virulent flu crossed Canada by rail, from coast to coast.