After being declared a pandemic, Swine Flu does not become more dangerous. Epidemics on a global scale - struck three times in the 20th century and once in the 21st century, varying widely in their severity. The classification of swine flu as a pandemic does not mean that it has become more deadly, just that its geographical spread is now truly global.

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

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 affected the elderly. In all, about 2 million people died around the world.

- 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.

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