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The influenza pandemic of 1918, known as the Spanish Flu, was one of the deadliest and widespread pandemics in human history. The scale of the outbreak, as well as limitations in technology, medicine and communication, create difficulties when trying to uncover accurate figures relating to the pandemic. Estimates suggest that the virus, known as the H1N1 influenza virus, infected more than one quarter of the global population, which equated to approximately 500 million people in 1920. It was responsible for roughly 25 million fatalities, although some projections suggest that it could have caused double this number of deaths. The exact origins of this strain of influenza remain unclear to this day, however it was first noticed in Western Europe in the latter stages of the First World War. Wartime censorship in Europe meant that the severity of the pandemic was under-reported, while news outlets in neutral Spain were free to report openly about the impact of the virus; this gave the illusion that the virus was particularly strong in Spain, giving way to the term "Spanish Flu".

Effects of the virus

By late summer 1918, the pandemic had spread across the entire continent, and the H1N1 virus had mutated into a deadlier strain that weakened the infected's immune system more than traditional influenzas. Some studies suggest that, in contrast to these traditional influenza viruses, having a stronger immune system was actually a liability in the case of the H1N1 virus as it triggered what is known as a "cytokine storm". This is where white blood cells release proteins called cytokines, which signal the body to attack the virus, in turn releasing more white blood cells which release more cytokines. This cycle over-works and greatly weakens the immune system, often giving way to other infections; most commonly pneumonia in the case of the Spanish Flu. For this reason, the Spanish Flu had an uncommonly high fatality rate among young adults, who are traditionally the healthiest group in society. Some theories for the disproportionate death-rate among young adults suggest that the elderly's immune systems benefitted from exposure to earlier influenza pandemics, such as the "Asiatic/Russian Flu" pandemic of 1889.

Decrease in life expectancy

As the war in Europe came to an end, soldiers returning home brought the disease to all corners of the world, and the pandemic reached global proportions. Isolated and under-developed nations were especially vulnerable; particularly in Samoa, where almost one quarter of the population died within two months and life expectancy fell to just barely over one year for those born in 1918; this was due to the arrival of a passenger ship from New Zealand in November 1918, where the infected passengers were not quarantined on board, allowing the disease to spread rapidly. Other areas where life expectancy dropped below ten years for those born in 1918 were present-day Afghanistan, the Congo, Fiji, Guatemala, Kenya, Micronesia, Serbia, Tonga and Uganda. The British Raj, now Bangladesh, India and Pakistan, saw more fatalities than any other region, with as many as five percent of the entire population perishing as a result of the pandemic. The pandemic also had a high fatality rate among pregnant women and infants, and greatly impacted infant mortality rates across the world. There were several waves of the pandemic until late 1920, although they decreased in severity as time progressed, and none were as fatal as the outbreak in 1918. A new strain of the H1N1 influenza virus did re-emerge in 2009, and was colloquially known as "Swine Flu"; thankfully it had a much lower fatality rate due to medical advancements across the twentieth century.