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Swine flu is a kind of influenza common in pigs. And sometimes humans catch it too. Human cases of swine flu are not that common. But occasionally, many people are infected at the same time. This is called an "epidemic." When an epidemic spreads over a whole country or the world, it is called a "pandemic."

On April 30, 2009, the World Health Organization (WHO) made an announcement. The WHO said that swine flu had reached pandemic Level 5. There are 6 Levels (or Phases)...