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?
That question weighs heavily on the minds of influenza scientists and public health planners as they prepare for the next big outbreak. And there will be a next time. Flu viruses mutate constantly. Occasionally those changes result in viruses so different from what our immune systems have seen before that they are able to trigger global waves of disease, or pandemics. Someday there may be a vaccine that can fend off all subtypes of influenza, but such a vaccine remains a dream for now. So new viruses can and will come at us from birds or pigs or other animals. The best we can do is try to spot new invaders soon enough to get a jump on producing vaccines against those particular bugs, to shorten the time from first infections to mass immunizations. No one wants a repeat of 2009, when a vaccine arrived about the time the outbreak was peaking and public interest was waning.