In 1918, the Spanish flu made its way across the globe and lingered for nearly two years. The pandemic left some 20 to 50 million dead worldwide, with roughly 675,000 people dead in the United States. One hundred years following the outbreak, we speak with historian Kenneth C. Davis about how the virus became so deadly and widespread. He's the author of the new book, "More Deadly Than War: The Hidden History of the Spanish Flu and the First World War".