I'm reading "The Great Influenza" by John M. Barry right now, and it is a remarkable book about the influenza pandemic of 1918. It describes the background of numerous factors that went into the pandemic's occurrence, including the history of medicine in the US, military history, the events of the "Great War" that affected the movement of troops (and therefore the movement of the virus), virology, etc. It's a fascinating account of a virus that affected nearly every family in the US (and worldwide). Roughly half the population became ill, and 10-20% of those who fell ill died. According to the book, approximately 675,000 Americans died from the 1918 flu pandemic, and that was with a third of the population we have today. In current population terms, with ~287 million Americans, that would be equivalent to losing 1,750,000 people to the flu.

Some people fell ill and died remarkably quickly - one man asked for directions in a perfectly normal tone of voice, and then fell over dead. One man got on a streetcar for a three mile trip and while on it, the conductor, driver, and 6 passengers died, and they'd been perfectly normal until moments before they died. He walked the rest of the way home.

I highly recommend "The Great Influenza" - it's a fascinating and frightening read.