Technically, the term swine flu refers to influenza in pigs. Occasionally, pigs transmit influenza viruses to people, mainly to hog farm workers and veterinarians. Less often, someone infected occupationally passes the infection to others.
The human respiratory infection caused by the influenza virus H1N1 — popularly known as swine flu — was first recognized in spring 2009, near the end of the usual Northern Hemisphere flu season.
A few months after the first cases of swine flu 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, until the pandemic was declared over in August 2010. Currently, H1N1 is still circulating in humans as a seasonal flu virus and is included in the seasonal flu vaccine.
Another strain of swine flu — H3N2 variant — has also made its way into humans. This flu contains a gene from the H1N1 virus. So far, the virus has only been transmitted through human exposure to swine, not from human to human.
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