The 1918 flu pandemic (commonly referred to as the "Spanish Flu") was an influenza pandemic that spread to nearly every part of the world. It was caused by an unusually virulent and deadly influenza A virus strain of subtype H1N1. Historical and epidemiological data are inadequate to identify the geographic origin of the virus. Most of its victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients. The flu pandemic has also been implicated in the sudden outbreak of encephalitis lethargica in the 1920s. The pandemic lasted from March 1918 to June 1920, spreading even to the Arctic and remote Pacific islands. It is estimated that anywhere from 50 to 100 million people were killed worldwide which is from three to seven times the casualties of the First World War (15 million). An estimated 50 million people, about 3% of the world's population (approximately 1.6 billion at the time), died of the disease. An estimated 500 million, or 1/3 were infected. Although most think the Spanish Flu is what killed those 50 million people, it was, in most cases, actually Pneumonia that killed them, not the Spanish Flu. The Spanish Flu only weakened the immune system, while the Pneumonia slowly suffocated them to death.
Although the first cases of the disease were registered in the continental U.S, and the rest of Europe long before getting to Spain, the 1918 Flu received its nickname "Spanish flu" because Spain, a neutral country in WWI, had no special censorship for news against the disease and its consequences. Hence the most reliable news on the disease came from Spain, giving the false impression that Spain was the most—if not the only—affected zone.
The Spanish Flu may have been the worst and worst to come of all of the H1N1 outbreaks, if the 2009 global pandemic (see Influenza A (H1N1) virus "swine flu") doesn't get worse.