The World Health Organization has called the swine flu outbreak spreading around the world a "public health emergency of international concern."
Health workers worldwide are racing to prevent what may potentially become a pandemic.
An influenza pandemic occurs when a new virus appears against which the human population has no immunity, according to the WHO.
Christine Layton, a public health expert who specializes in influenza at research institute RTI International, told CNN the swine flu has "pandemic potential."
"Unlike the avian flu that people were concerned about a few years ago, a lot more cases are occurring in a lot more different places," she said. 
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Sources: World Book Online, U.S. Department of Health and Human Services
All About Influenza • Centers for Disease Control and Prevention
