The 2009 influenza pandemic appeared to come out of nowhere. It started as what seemed like a lethal outbreak in Mexico, then spread north of the border. By the time health officials learned that the virus responsible for the alarming explosion of cases was new and an infection threat to most of humankind, they had no way to keep it from spreading around the world. By a stroke of luck, symptoms were mild in the vast majority of cases. What if next time we are not so lucky?