Normally, swine flu bugs don't infect people. When they do, it's been in people who have direct contact with pigs; historically, there's such a case every year or two in the U.S.
Between Dec. 2005 and Feb. 2009, there were12 human swine flu infections in the U.S. -- about four a year.
It's possible this uptick was due to improved reporting systems, but the U.S. Centers for Disease Control says "genetic changes in swine flu viruses and other factors might also be a factor."
Since March 2009, a number of confirmed human cases of a new strain of swine influenza A (H1N1) virus infection in the U.S. and internationally have been identified.
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