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→The nature of the challenge
=== The nature of the challenge ===
In the past 300 years, flu pandemics have happened once every 10-50 years. Each one starts when a flu virus present in non-human animals mutates or swaps genes with another virus, and a fresh human-adapted virus emerges. Many people without specific immune experience are infected in two or more epidemic waves. Once the virus is not new for most people, we can no longer call it a pandemic and it becomes the new seasonal flu.
Pandemics are very different from each other. The 1918 pandemic, by most estimates, killed 50 million from a 2000 million global population [http://www.cdc.gov/ncidod/eid/vol12no01/05-0979.htm]. The ones that started in 1957, 1968 and certainly 2009 were very different. The next one? There are some known candidate viruses, but science can’t yet predict which virus will cause it, how contagious it will be, what will be the proportion of severe and lethal cases, or which age groups will be most affected.