Between 50 and 100 million people are reported to have died as a direct consequence of the infection, which was caused by a particularly virulent avian influenza virus of the subtype H1N1. The influenza strain responsible for the Spanish flu is understood to have been particularly virulent, with estimates on case fatality rate as much as 25 times higher than other influenza pandemics.
The first official cases of the outbreak were reported to have occurred in March 1918, among armed services staff in the US, though earlier infections are thought to be likely.
With the onset of the First World War, the H1N1 virus soon made its way across Europe, quickly becoming a pandemic.
Where did the Spanish flu originate?
The exact point of origin for this strain of H1N1 virus isn't known.
Some speculate the virus's animal hosts are among Chinese bird livestock, potentially emerging with an outbreak of a respiratory disease in Shanxi province in 1917. Chinese labourers might then carried the pathogen to the west as they were brought in to assist behind the front line of the war.
While it seems typical of most seasonal waves of influenza virus that emerge out of the eastern hemisphere, others point out the mortality rate of the Chinese labourers lagged behind deaths in other nearby military camps, making this scenario less likely.
Why was the virus called the Spanish flu?
As the flu virus made its way across Europe in late 1918, nations on both sides of the conflict were tight-lipped about how this deadly disease was taking the lives of combatants, fearing how it might affect morale.
Spain had no such qualms. Being a neutral nation, its media was comfortable reporting on the epidemic's spread, creating an impression that they alone were dealing with this dreaded influenza.
Believing the illness had spread from the north with movement of labourers across the border, Spanish news sources referred to the virus as the French flu.
What happened to the Spanish flu?
In the early years of the flu's emergence, communities followed strict rules on wearing masks and following social distancing. While adherence to these practices helped reduce its spread, waves of the virus continued to take lives for several years - with the second wave being the deadliest.
Infections bestowed a level of immunity on many populations, though the high death rate also made it difficult for the virus to spread efficiently - although because of WWI it spread quite rapidly among soldiers and hospitals.
As seasons passed, less aggressive strains of the H1N1 virus evolved. These spread as easily, but without harsh symptoms that risked killing their host.
Over time, the virus that gave us the Spanish flu pandemic became a milder, less deadly version as the original virus found it harder to find a host.
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