Spanish flu was one of the deadliest pandemics in human history, and in recent weeks, it has been used as something of a measuring stick when assessing the ongoing impact of coronavirus.
From 1918 to 1920, H1N1 influenza A infected 500 million people around the world, including people on remote Pacific islands and in the Arctic.
The 1918 virus was also ‘novel’ (a new virus never before seen), nobody had immunity to it and its spread through respiratory droplets made it highly infectious, much like Covid-19.
But how does it stack up when compared to the havoc that continues to be wreaked by Covid-19 today, almost a year after reports of a mysterious, flu-like disease began emanating from the Chinese city of Wuhan?
Read more: What was the biggest pandemic in history? From Spanish flu to swine flu - what happened during pandemics of the past
Here's everything you need to know.
Where did Spanish flu come from?
Despite its name, the first recorded cases and deaths from Spanish flu were in the US, France, Germany and the UK.
Since World War I was already hammering the global economy and causing mass physical and mental strain to people across the world, most countries opted to censor their Spanish flu epidemics.
Spain remained neutral in the conflict, and so newspapers there were free to report on the virus’ effects, including the infection of the county’s King Alfonso XIII.
This created the false impression that the virus originated in Spain, and the country was especially hard hit.
Which pandemic has claimed the most lives?
On 29 September 2020, the coronavirus death toll topped one million, a staggering number of lives lost to the pandemic. But how does that compare with Spanish flu?
Of the two, the current Covid-19 outbreak is by far the least deadly, and Spanish flu is perhaps the costliest pandemic in recent history.
It’s thought the Spanish flu could have been up to 50x as deadly, with death count estimates ranging between 17 and 50 million people.
That's more people than were killed in World War I, and about a third of the world's population at the time.
Who did it affect?
Older, more vulnerable people are more at risk from developing serious complications that can lead to death from Covid-19, but Spanish flu had an unusually high mortality rate for young adults.
That's thought to be because the virus triggered a ‘cytokine storm’, a physiological reaction which causes the immune system to release an excessive amount of pro-inflammatory molecules, which in turn can ravage the body’s defenses.
Was there a second wave of Spanish flu?
The Spanish flu pandemic is often used as a warning against underestimating Covid-19, and those campaigning for stricter lockdown restrictions often point to Spanish flu as an example of what could happen if governments get it wrong.
Spanish flu did indeed experience a second wave, one which was undoubtedly worse than the first as the virus began to mutate and get stronger.
Symptoms were much more severe than were seen during the first wave – patients could die within days or even hours of contracting the virus.
It had become more infectious too, and it’s thought an outbreak originating from a single parade held in Philadelphia in September 1918 was responsible for over 12,000 deaths.
But it didn’t stop there. A third wave struck in January 1919, rocking Britain with hundreds of thousands of fatalities.
While this wave was overall less deadly than the second, it was still much worse than the first.
Over two years on from the first reports of Spanish flu, the world was still battling against it as a fourth wave emerged, with cases recorded as late as April 1920 in places like Spain, Denmark, Finland, Germany, and Switzerland.
What were the symptoms?
The majority of the infected experienced only the typical flu symptoms of sore throat, headache, and fever, especially during the first wave.
However, during the second wave the disease was much more serious, often complicated by bacterial pneumonia.
For comparison’s sake, the main symptoms of coronavirus are a high temperature, a new, continuous cough and a loss or change to your sense of smell or taste.
Why was Spanish flu so bad?
As mentioned, Spanish flu made its appearance at the tail end of World War I, when many country’s economies and resources had been pounded by the global conflict.
That meant medical camps and hospitals – already crowded following the war – were unable to reach their usual standards of hygiene and patient care, which promoted the spread of the virus beyond what would normally be expected.
Coupled with less than adequate conditions on the front line of the war for military personnel, and the high level of troop movement around the world, the circumstances were perfect for the virus to spread.