When a new influenza virus appears, against which the human population has no immunity, a pandemic may occur. This happened in 1918 (the “Spanish flu”, caused by an H1N1 subtype), in 1957 (the “Asian flu” caused by an H2N2 subtype) and in 1968 (the “Hong Kong flu”, caused by an H3N2 subtype). Of all pandemics, the one that began in 1918 is generally regarded as the most deadly disease event in human history. At least 40 million and likely closer to 100 million deaths worldwide have been attributed to the virus, most of them occurring in the 16-week period between September and December 1918. The virus struck hardest in the young and healthy, where their rapid immune response actually became their downfall. Though the 1918 pandemic has been the most dramatic example of the killing potential of influenza, there have been two other pandemics in the last 100 years. In 1957, an H2N2 virus appeared in China. This “Asian flu” quickly swept through the population, replacing the previously-circulating H1N1 virus and killing 1-4 million people worldwide. Similarly, in 1968, an H3N2 virus emerged from Hong Kong to replace the H2N2 virus. This pandemic resulted in 1-2 million deaths. The H1N1 serotype re-surfaced in 1977, and currently, H3N2, H1N1 and reassortant H1N2 viruses are circulating in the human population.