A new strain of bird flu has infected 130 and killed 40 people in China already this year. Take a look at history's four main flu outbreaks.
Unlike earlier pandemics and seasonal flu outbreaks, the 1918 pandemic flu saw high mortality rates among healthy adults. In fact, the illness and mortality rates were highest among adults 20 to 50 years old. The reasons for this remain unknown.
Keystone Resort/Ben Lindbloom
Jessica Rinaldi / Reuters
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