The Spanish flu was a terrible worldwide epidemic that killed between 50-100 million people in an 18 month period throughout 1918 and 1919. This classifies it as a 5 on the Pandemic Severity Index, meaning more than 2% people who were infected died. The Spanish flu resulted in the death of 2.5-5% of the world population at the time it struck, killing more than World War I, which it occurred immediately after. The Spanish flu was in the same category of severity as the Bubonic Plague, which, when it struck as the Black Death, killed about 75 million people, 25-50 million of them in Europe.
The Spanish flu was caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1. In contrast to most influenza outbreaks in history, the Spanish flu struck people down in their prime of life, rather than picking off the old and young. People with weaker immune systems, such as children and middle aged adults, had lower mortality rates, while young adults had the highest mortality rates.
The distribution pattern of deaths has led scientists to argue that the Spanish flu killed because of an excessive immune response, called a cytokine storm. In a cytokine storm, the immune response is so overwhelming that the overabundance of immune cells, such as macrophages, can clog local tissues, causing the buildup of fluids and eventually fatal damage. Cytokine storms are normally rare, and are thought to be caused as a reaction of the immune system to a novel and highly pathogenic invader.
In comparison to a more typical case of the flu, which kills 0.1% of those infected, the Spanish flu killed between 2-20% of sufferers. The primary cause of death was from a secondary infection of the lungs, bacterial pneumonia. The secondary cause of death was from the virus itself, which caused massive hemorrhages and edema in the lungs.
Genetic material from the Spanish flu virus has been recovered from the corpse of a flu victim in Alaskan permafrost, a woman who had collapsed in the wilderness after being struck down by the disease. This genetic material has been used to recreate the virus from scratch and sequence its entire genome, which has been published on the Internet. Some technologists, such as inventor Ray Kurzweil and Sun Microsystems co-founder Bill Joy, have expressed dismay at this development.