One hundred years ago, a world recovering from a global war that had killed some 20 million people suddenly had to contend with something even more deadly: a flu outbreak.

The pandemic, which became known as Spanish flu, is thought to have begun in cramped and crowded army training camps on the Western Front. The unsanitary conditions – especially in the trenches along the French border – helped it incubate and then spread. The war ended in November 1918, but as the soldiers returned home, bringing the virus with them, an even greater loss of life was just around the corner; between 50 million and 100 million people are thought to have died.

The world has suffered many pandemics in the years since – at least three serious flu outbreaks among them – but no pandemic has been as deadly, nor as far-reaching.

As the world reacts to a headline-grabbing – yet far, far less deadly – outbreak of Covid-19, caused by a new coronavirus, BBC Future looks back to our 2018 special marking the 100th anniversary of Spanish Flu to see what we learned from one of the most devastating diseases in recent history.

Pneumonia is often the killer

Many of the people dying from Covid-19 are succumbing to a form of pneumonia, which takes hold as the immune system is weakened from fighting the virus.

This is something that it shares with Spanish flu – though it must be said that the death rate from Covid-19 is many times lower than that of Spanish flu. Older people and those with compromised immune systems – who make up the majority of those who have been killed by the disease so far – are more susceptible to infections that cause pneumonia.

Read more: The flu that changed the world

Few places escaped

Air travel was in its infancy when Spanish flu struck. But there are few places on Earth that escaped its horrific effects. Its passage across the world was slower, carried by railway and passenger steamer rather by airliners. Some places held out for months, or even years, before the flu arrived and wreaked its terrible toll.