"We're seeing a big increase, and we think it's a real increase," the CDC's Dr. Joseph Bresee said Thursday.
The CDC said the victims, most of them children, are believed to have been infected after contact with pigs at county fairs, the Los Angeles Times reported.
Cases were confirmed in Indiana, Ohio, Illinois and Hawaii.
Most of the illnesses have been mild, the CDC said, with just two people hospitalized and no fatalities.
However, CDC officials are advising anyone planning to attend an agricultural fair to take precautions.
Such fairs, which are common during summer, offer prime conditions for the flu to spread, Richard Webby, a flu researcher at St. Jude Children's Hospital in Memphis, said.
"You bring together these fair pigs. I'd assume a lot are not vaccinated. ... You get this burst where the virus transmits rapidly among pigs," he said. "At the same time, you're getting more people walking through the barns than you would in a commercial farm."
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