High Quality Content by WIKIPEDIA articles! The 1918 flu pandemic (the "Spanish" flu) was an influenza pandemic, and the first of the two pandemics involving H1N1 influenza virus (the second was the 2009 flu pandemic, an outbreak of Swine Flu). It was an unusually severe and deadly pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin. Most victims were healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or weakened patients. The flu pandemic was implicated in the outbreak of encephalitis lethargica in the 1920s.