The 1918 flu pandemic (the "Spanish" flu) was an influenza pandemic

Influenza pandemic

An influenza pandemic is an epidemic of an influenza virus that spreads on a worldwide scale and infects a large proportion of the human population. In contrast to the regular seasonal epidemics of influenza, these pandemics occur irregularly, with the 1918 Spanish flu the most serious pandemic in...

, and the first of the two pandemics involving H1N1 influenza virus (the second was the 2009 flu pandemic

2009 flu pandemic

The 2009 flu pandemic was an influenza pandemic, and the second of the two pandemics involving H1N1 influenza virus , albeit in a new version...

, an outbreak of Swine Flu). It was an unusually severe and deadly pandemic that spread across the world. Historical and epidemiological

Epidemiology

Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...

 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.