Spanish Flu (1918-1919): "It is estimated that about 500 million people or one-third of the world's population became infected with this virus. The number of deaths was estimated to be at least 50 million worldwide with about 675,000 occurring in the United States." (Source: www,cdc.gov) And that was before the massive international travel we have today. Oh, and our "over sanitized" germophobic population who have little or no built up immunity system to deal with such things as germs and viruses.