(Associated Press) -- Flu pandemics -- epidemics on a global scale -- have struck three times in the 20th century and once in the 21st century, varying widely in their severity. Here's a look at the world's flu pandemics.

- The 1918 Spanish flu: This pandemic may have been the deadliest outbreak ever, with experts estimating it killed between 20 million to 50 million people worldwide. It apparently began in the United States, but attracted attention when it started having a big impact in Spain.

- The 1957 Asian flu: This new strain was identified in China and caused two waves of illness, the first hitting primarily children while the second mostly affecting the elderly. In all, about 2 million people died around the world.

- The 1968 Hong Kong flu: This pandemic spread globally over two years from Hong Kong, having its biggest effect on the elderly. It killed about 1 million people.

- The 2009 swine flu: Swine flu has infected nearly 30,000 people in 74 countries so far, killing at least 144 people. The pandemic declared Thursday is the first of the 21st century.

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