If you are reading this article, you have probably lived through at least one global flu pandemic – one just as contagious as the deadly 1918 strain. There was the 1957 outbreak (the so-called ‘Asian flu’) and the ‘Hong Kong flu’ in 1968. Forty years later, in 2009, there was ‘swine flu’.

Each pandemic had similar origins, emerging, one way or another, from an animal virus that evolved to be to able pass between humans. Yet the death tolls were barely comparable. Between 40 and 50 million are thought to have died from the 1918 strain – compared to two million for the Asian and Hong Kong influenzas, and 600,000 for the 2009 swine flu, both of which had a mortality rate of less than 1%.

The human cost of the 1918 pandemic was so great that many doctors continue to describe it as the “greatest medical holocaust in history”. But what made it so deadly? And could that knowledge help us prepare for a similar pandemic today?

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An understanding of these pandemics would be impossible without a recognition of the huge leaps in medicine over the 20th Century. Doctors in 1918 had only just discovered the existence of viruses. “And they certainly didn’t know that this was a virus causing these diseases,” says Wendy Barclay at Imperial College London. They were a long way from the anti-viral medications and vaccines that can now help to stem the spread and promote a quicker recovery.

Many flu deaths are also caused by secondary, bacterial infections that take root in the weakened body, leading to pneumonia. Antibiotics like penicillin – discovered in 1928 – now allow doctors to reduce that risk, but in 1918 there was no such treatment. Nor did they have vaccines, which now help to protect those who are most at risk. “Our health care infrastructure and diagnostic and therapeutic tools are so much more advanced,” says Jessica Belser, who works at the influenza division of the US Centers for Disease Control and Prevention.

Besides the lack of basic medical tools in 1918, deaths would have also been a direct result of the appalling living conditions at this tragic time in human history. The trenches would have been the perfect breeding grounds for infections among the World War One soldiers. “The virus emerged when populations, which previously had little contact with each other, were brought together on the battlefield,” says Patrick Saunders-Hastings at Carleton University in Ottawa. “And on a lot of cases they were dealing with other injuries and they were under-nourished.” Vitamin B deficiencies, in particular, have been noted to increase mortality rates in later pandemics, he says.