A common viral infection, Influenza can be dangerous if the victim is old, chronically ill or young, it can even kill. There has been 5 known Influenza Pandemics:
1889-1890 - H2N2 (Russian or Asiatic) Strain
1900 - H3N8 Strain
1918-1920 - H1N1 (Spanish) Strain
1957-1958 - H2N2 (Asian) Strain
1968-1969 - H3N2 (Hong Kong) Strain

The Spanish Pandemic was very devastating as it is thought there were more casualties from the pandemic than there were from World War I. The N5N1 (Avian or Bird) Influenza strain is rapidly spreading throughout countries from migrating birds.