Deadly Spanish Flu Remains a Puzzle As scientists prepare for the coming flu season, they are still trying to understand why the so-called Spanish flu of 1918 was so deadly. At least 20 million people died worldwide, including more than 500,000 in the United States. Now, a team of scientists has used DNA sequences from victims of the 1918 flu to create a flu strain that kills mice much the way the 1918 flu killed humans. They say experiments with that strain are helping them understand how the 1918 strain killed so many people. Hear NPR's Jon Hamilton.