The as called Spanish flu epidemic of 1918 has been described to date as one of the most deadly pandemics in history. Occuring during world war 1, deaths from the virus may have been difficult to estimate. Anywhere between 17 and 100 million according to one source.
Said to have first been noticed among U.S. military personnel who had returned from Europe, there is no evidence that it started in Spain who were neutral during the war. Flu rarely attack young adults but war and deprivation made the soldiers vulnerable, who were hard hit.
The so called Spanish influenza of 1918, along with the swine flu outbreak of 2009 are said to have been caused by a virus named H1N1. Similar to seasonal flu, H1N1 virus affected mammals and also salmon. What was different about this virus was a glycoprotein found on its surface named haemoglutinin. It has been described as a fusion protein which is believed to be integral to its high rate of infectivity.
The Spanish influenza outbreak was said to have lasted from Spring 1918 until 1920 but became thought of as more and more benign as individuals became more immune to it. Individuals started to look after themselves and alternative healthy ideas came to the fore.