The influenza outbreak of 1918–1919 carried off between 20 to 40 million people worldwide. The Spanish flu outbreak differed significantly from other influenza (flu) epidemics. It was much more lethal, and it killed a high proportion of otherwise healthy adults. Most flu outbreaks kill only the very young, the elderly, and people with weakened immune systems. Scientists and public health officials have been trying to learn more about Spanish flu in the hopes of preventing a similar outbreak.

The Spanish flu virus caused one of the worst pandemic of an infectious disease ever recorded. And while the threat of many infectious diseases, including tuberculosis and smallpox, have been contained by antibiotics and vaccination programs, influenza remains a difficult disease. There are worldwide outbreaks of influenza every year, and the flu typically reaches pandemic proportions (lethally afflicting an unusually high portion of the population) every 10–40 years. The last influenza pandemic...