The Spanish Flu probably didn't originate in Spain. It was given that name during World War I, when Spain, a neutral country, began reporting on how bad the flu was in their home country. This lead to people calling it a false name: The Spanish Flu. The Spanish Flu was a massively up-scaled H1N1 influenza A virus outbreak that originated in an unknown location due to pacts of secrecy taken by various world powers to keep their infected counts under wraps. Due to the war-torn globe, the Spanish Flu was able to run rampant through the population of the Earth, infecting as many as 500 million people, almost a third of the globe's population. This was because of poor hygiene amongst the soldiers on the battlefield, combined with malnourishment and overcrowding in medical camps and hospitals, the Spanish Flu was a different beast.
Strangely enough, the H1N1 virus did not kill the usual suspects: the very young and the very old. Instead, it targeted young adults, triggering a Cytokine storm, which is an over-reaction of the body, which can lead to death. This mainly affected young adults because their immune systems were stronger than their counterparts'.