Spanish flu is estimated to have killed between 50 million and 100 million people when it swept the globe in 1918-19 – more than double the number killed in the first world war. Two-thirds of its victims died in a three-month period and most were aged 18-49. So what lessons has the world’s deadliest pandemic taught us?

1) How not to name a pandemic

In spring 1918, soldiers in Europe on both sides starting suffering from a new type of influenza. But if the illness seemed worse than seasonal flu, the squalid conditions of the trenches were blamed and, not wanting to admit any potential weakness, Britain, France and Germany kept the outbreak a secret. It was only when the disease hit Spain, a neutral country, that the first accurate reports emerged giving the pandemic its name.

Misidentifying illnesses as being from one country or one ethnic group is a common theme (the 2009 swine flu outbreak was initially labelled “Mexican flu”) and often leads to stigma and racism. In Spain “the name caused offence for a long time after”, said Prof Julia Gog, a mathematician who researches flu dynamics at the University of Cambridge.

“When we give talks we’re careful to call it 1918 influenza – there’s no way its origin was Spain. There’s a joke that when an epidemic is said to emerge first in a place, it almost certainly didn’t. Except Wuhan might actually be right.”

2) Tell the truth and warn the public

The Spanish flu hit during the first world war, so authorities were unusually keen to avoid further social disruption or blows to national morale. Much of the pandemic was characterised by increasingly untenable reassurances that the Spanish flu was not something to be overly concerned about. In June 1918, just before the UK felt the full force of the outbreak, the Daily Mail advised readers that flu was no worse than a cold and that people should not have “any great dread” but “maintain a cheery outlook on life”. The Times initially adopted a casual, jokey tone before growing critical of official complacency.

“Epidemics all follow this similar arc where people deny or dismiss the threat until it becomes impossible to ignore any more,” said Mark Honigsbaum, a medical historian at City, University of London. The way coronavirus is being played down by some leaders is concerning, he said, and even throwaway comments that appear to undermine the seriousness of the threat can set the tone for how people respond to advice.

“You still have politicians like Boris Johnson saying they’re happy to shake hands. Just stop shaking hands,” said Honigsbaum. “We need to take this a little bit more seriously. I’m shocked by the complacency at the political level.”

3) Uncontrolled movement can lead to tragic outcomes

The unusual circumstances of the first world war reversed the normal pattern of human movement in a pandemic. Instead of the sickest people staying put in bed, many were sent home from the frontline, which may have contributed to the rapid spread of the disease. And many countries struggled to balance national war interests with public health.

In New Zealand, which was slow to clamp down on ships entering and leaving its ports, nearly 1% of the population died within two months in late 1918, triggering even worse outbreaks in Pacific territories such as Western Samoa, where 30% of men, 22% of women and 10% of children died.

By contrast, the coronavirus outbreak has prompted draconian clampdowns on civil life. The World Health Organization has credited the response in China, where entire cities were placed in lockdown and schools and work suspended, with averting hundreds of thousands of cases of Covid-19.

4) The potential for a second wave

The first wave of Spanish flu in spring 1918 wasn’t that bad. But by August, when a second wave spread from France across Europe, the US and much of the globe, the virus had mutated to a far deadlier form and appeared to hit hardest in regions that had not already been exposed, meaning people had lower immunity.

Flu viruses are fundamentally different from coronaviruses in that they are constantly shuffling their genomes, which means they rapidly morph from one strain into another – that’s why flu vaccines are needed annually. Coronaviruses tend to be genetically fairly stable and so scientists don’t expect a sudden shift in the mortality rate of Covid-19. But the question of whether coronavirus will disappear, reappear in waves or simmer in the background as an endemic illness still remains to be answered.

Viruses where transmission is strongly affected by temperature and humidity often come in waves and initially it was expected that Covid-19 might die down in the spring and possibly reappear next winter. But the geographic spread is already raising questions about this.

“Iran right now, Italy right now, they’re all warmer than the UK, so the argument that we’ll be safer as we move towards the summer doesn’t make sense,” said Gog. “How transmission depends on temperature and humidity is unknown, but it’s possible it may not be a two-wave thing, at least not due to climate.”

Whether we see a second wave will also depend on whether people who have been infected once have long-term immunity, which would suppress re-emergence in the community. Reports of a Japanese woman getting the virus twice raised the prospect that immunity may wane in the weeks or months after recovery, which some think could make a second wave more likely. This is something experts are tracking closely as case numbers continue to rise.