Doctors in the early 1900s dismissed Spanish flu as a 'minor infection' just years before it killed 50million people, according to scientists.
Countless lives could have been saved if medics had taken it seriously and worked out how to stop the virus before the disastrous outbreak in 1918, researchers say.
A study has found there were investigations as early as 1915 into a mysterious illness which was killing World War I soldiers in France and England.
But doctors didn't notice it was a form of flu, according to research by a flu expert and military historian, and missed an opportunity to start a vaccination programme.
Some 500million people were infected with Spanish flu after the pandemic broke out in 1918 (Pictured: Patients on a hospital ward in Fort Collins, Colorado in 1918)
The global pandemic, which coincided with the First World War which helped it to spread around the world, would go on to kill more than 50million people (Pictured: Sick patients in a makeshift hospital in Oakland, California in 1918)
A medical group in Etaples, northern France, reported treating hundreds of people with an 'unusually fatal disease' causing 'complex' breathing problems in 1917.
And soldiers were being struck down with the virus across the Channel in Aldershot, England, in 1915 and 1916.
But doctors did not understand the severity of the illness nor predict the devastation it would go on to cause, according to research from Queen Mary University in London.
'We have identified long-neglected outbreaks of infection,' said Professor John Oxford, a leading virologist.
'Outbreaks which, judged as minor at the time, can now be seen as increasingly important, and a portent of the disaster to come.'
The 1918 Spanish flu pandemic was the worst in recent history and was triggered by a virus which originated in geese, ducks and swans.
Quickly spreading around the world, helped along by travelling soldiers fighting in the First World War, it infected an estimated 500million people in just two years.
Lives could have been saved if doctors had recognised the danger of Spanish flu when it first