Just one century ago, the world was in the grips of one of the deadliest pandemics in history. At least 50 million people – 3 percent of the world's population – were killed by the Spanish influenza pandemic that swept across the planet, considerably more lives lost than in World War I, which was also occurring at the time.

While much has changed since this chapter of the 20th century ended, the story of Spanish flu still holds a valuable lesson in not underestimating the pathogens we share Earth with. As a new study has detailed, the outbreak sharply highlights the importance of vaccination programs and the risks of complacency when it comes to communicable diseases in the globalized world. 

Writing in the journal Human Vaccines and Immunotherapeutics, a virologist and historian have detailed how the Spanish flu emerged from humble beginnings and took over the world in a matter of years. They argue that the Spanish flu may have emerged in Europe two years earlier than previously thought at sometime around 1915. For these two years, the virus was largely ignored and brushed off as a “minor respiratory infection”. 

By the time it was taken seriously, around 1918, the virus had mutated into a whole other kind of beast and it was too late to roll out effective vaccination programs.

"In essence, the virus must have mutated. It lost a great deal of its virulence but gained a marked ability to spread," study author Professor John S. Oxford, the UK's top expert on influenza, said in a press release. "Recent experiments with a pre-pandemic 'bird flu' called H5N1, deliberately mutated in the laboratory, have shown that as few as five mutations could have permitted this change to take place." 

"Once the virus is able to spread from human to human, disaster strikes. With a generation time of two to three days, from just three patients who were infected originally, a million infections can be caused in around 40 days, and this is probably exactly what happened in 1918-1919," Professor Oxford and Douglas Gill, a military historian, conclude in their paper.

The duo managed to track the origins of the virus using a combined approach of scientific methods, such as phylogenetics and molecular clock analysis, as well as historical documents, like newspaper articles and doctors’ reports.

Despite its name, it's been long established that the pandemic didn’t start in Spain. This name only arose because Spain was one of the few major powers to remain neutral during World War I. The Allies and Central Powers nations had installed tough wartime censorship in order to maintain good morale and control the narrative, while the Spanish media was free to report on the severity of the disease, giving the illusion the virus was particularly prevalent there.

Instead, this new analysis argues it started in England and France. They identify two detailed medical case reports from 1917 that explained how two groups of British soldiers had fallen sick in Etaples, France, and Aldershot, England, in 1916. In both instances, the disease was characterized by having a rapid progression from quite minor symptoms to death, but the diagnosis of a highly contagious flu strain was missed. This new study argues this was, indeed, the origins of Spanish flu.

If researchers had picked up on the severity of the virus in 1916, the study argues they would have had better grounds to start a vaccination program and curtail the influenza outbreak, potentially saving millions of lives.

"Something similar to what happened at the beginning of the 20th century could easily be repeated," Professor Oxford warned. "As a precaution, governments everywhere are stockpiling vaccines against the pneumococcus that usually develops as a secondary infection after the flu, and which causes fatalities on a very large scale."