A high-security containment facility in Atlanta, Georgia, keeps under lock and key an organism that in the course of a few months from 1918-1919 was responsible for more deaths than the number of people killed in the first world war. Vanished until 2005, the H1N1 strain of the influenza virus was brought back to life for the purpose of better understanding its catastrophic effect on the world’s population a century ago. But as Laura Spinney points out in Pale Rider, the resurrection is not thought universally to be a good idea.

The Spanish flu pandemic, which killed at least 50 million people, has remained something of an enigma, not only because scientists are still unsure about why it was so lethal, but because it’s a hugely significant world event that for decades seemed to have been largely forgotten.

Pale Rider sets out to change this; it’s both a saga of tragedies and a detective story. At the end of the war, among weakened soldiers and undernourished populations, flu appeared as a shocking and unheralded visitor, and spread with alarming speed. Its victims reported dizziness, fevers, lethargy and coughing up blood; for at least two out of every 100 people who caught the bug, the outcome, within weeks, was death; especially vulnerable were those aged between 20 and 40.

There were no tools available to identify the invisible agent of disease. We now know that the flu virus is a parasite that needs the host cell of another living organism in order to reproduce. In human hosts the desired cells are those lining the lung. Once reproduction is complete, the replicated virus must leave the host, be carried in the air, and infect another, if it is to thrive.

Flu’s symptoms represent the body’s immune response to the viral invasion. Antibodies are triggered and immune cells rush to the site of infection, releasing viral-killing cytokine chemicals; a consequent local inflammation is perhaps a small price to be paid by the host.

Virus H1N1, responsible for the Spanish Flu pandemic in 1918-19. Photograph: BSIP/UIG via Getty Images

But the flu virus also shows a propensity to mutate and generate variants; hence the need to manufacture new vaccines each year for seasonal outbreaks. In the case of the variant that came to be called the Spanish flu, the viral infection triggered a cytokine storm in some victims, a massive inflammatory response throughout the lungs that was hugely damaging to the host.

Spinney, an admired science journalist, conjures the drama of the Spanish flu when “your best chance of survival was to be utterly selfish” and “jealously guard your hoard of food and water, and ignore all pleas for help”. She charts the hysteria that broke out during the three waves of infection (the second being the most deadly) which swept across the globe.

There had been pandemics before, notably the Russian flu of the 1890s, which killed a million people, but nothing on this extraordinary scale. Bloated corpses clogged rivers; bells never ceased tolling for the dead; and smoke blocked out the sunlight for days as the unburied were cremated in huge funeral pyres.

The title of Pale Rider is taken from Revelation and the apocalyptic “Pale Horse, Pale Rider”, an African-American spiritual in which the rider’s name is Death. There are pitiful stories on every page, beginning with Guillaume Apollinaire, the French poet who, having survived the trenches and surgery to remove shrapnel from his brain, succumbed weeks later to flu. Doctors might save you from injuries inflicted on the battlefield but they had little to offer these victims. The great hopes for aspirin as a wonder drug that reduced fever were unfounded; penitent Christians in Zamora, Spain, were rewarded by their God with one of the highest mortality rates; gentiles in Odessa, borrowing the ancient Jewish ritual of the “black wedding”, chose couples to be married in the cemetery in the mistaken belief, as author Mendele Mocher Sforim wrote, that with “the knots tied amidst the graves of the parish dead, the contagion would at last stop”.

People in 1918 had been stunned by the devastation. Afterwards, they chose to protect themselves by not remembering

If the flu could not be treated, perhaps its spread might be curtailed. Countries attempted a policy of cordon sanitaire, closing their borders as they blamed each other for the virus’s origin; it had to have started somewhere. Over the decades, suspicion has fallen on three likely candidates associated with the movement of labourers and soldiers to the trenches: Shansi in China, Kansas’s Camp Funston and Étaples in France. None is yet to be proved. One thing is certain, though: the flu, despite its name, did not emanate from Spain.

Pale Rider is not just an excavation but a reimagining of the past. As the book progresses, the flu is cast increasingly as a character that crops up Zelig-like at important moments in history, altering the course of events previously unattributed to it. Spinney makes a compelling case, for instance, for considering Britain’s medical negligence in treating flu in its Indian colony as an eventual catalyst to India’s independence movement. She also argues that the flu, by producing sickly German soldiers, had an impact on the timing of the end of the first world war.

The mystery of why the Spanish flu was so vicious appeared partly solved when decades later a rare piece of preserved lung tissue from a victim of the 1918 pandemic was examined. Gene sequencing showed that its structure bore a close resemblance to the flu virus found in birds. Until the 1990s, it had been assumed that bird flu virus could not be transmitted to humans. But the virus had mutated and jumped species. Its foreignness meant that the human immune system was vulnerable and, unprepared for the shock, triggered a violent response.

People in 1918 had been stunned by the devastation. Afterwards, they chose to protect themselves by not remembering. So severe was the impact of this “mother of all pandemics” on the community of Alaska Natives of Bristol Bay that its elders advised survivors to nallunguarluku, “to pretend that it didn’t happen”. In many other places, decisions were taken to opt for silence, if less explicitly.

The Centers for Disease Control and Prevention believes another pandemic is likely, if not inevitable. Its researchers in Atlanta reversed the genetics system to generate the 1918 flu virus. While the anxiety has been expressed that it is a blueprint for bio-terrorists “to develop and unleash a devastating pandemic on the world”, the CDC maintains that the virus’s reconstruction militates against “the potential threat posed by new strains”.

The renowned virologist John Oxford concurs: “H1N1 has a proven capacity to kill,” he says, “and we don’t need to be sitting here taking it like they did in 1918.” Spinney has ably lent her pen to the cause.

Colin Grant’s A Smell of Burning: The Story of Epilepsy is published by Cape.

• Pale Rider: The Spanish Flu of 1918 and How It Changed the World is published by Cape. To order a copy for £15 (RRP £20) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.