The respiratory infection popularly known as swine flu is caused by an influenza virus first recognized in spring 2009, near the end of the usual Northern Hemisphere flu season.
The new virus, 2009 H1N1, spreads quickly and easily. A few months after the first cases were reported, rates of confirmed H1N1-related illness were increasing in almost all parts of the world. As a result, the World Health Organization declared the infection a global pandemic. That official designation remained in place for more than a year.
Technically, the term "swine flu" refers to influenza in pigs. Occasionally, pigs transmit influenza viruses to people, mainly hog farm workers and veterinarians. Less often, someone infected occupationally passes the infection to others. You can't catch swine flu from eating pork.
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