Dallas County officials said Friday that any patient with upper respiratory ailments should be tested for swine flu. The warning comes after a strain of swine flu was diagnosed in five people in California and two in Texas.
None of the seven people that have been diagnosed had been in contact with pigs, which is how people usually catch swine flu, and all recovered from the illness.
"Because the strain is not widely studied yet, we don't know how it's transmitted, entirely," said Dr. Edward Goodman, director of Infection Control at Texas Health Dallas.
The U.S. Centers for Disease Control and Prevention is checking people who have been in contact with the seven confirmed cases, who all got sick between late March and mid-April.
"It's a little surprising to see seven cases in the span of a few weeks, where there were only 12 cases in the last two years," Goodman said.
The Texas cases are 16-year-old boys who are friends and live in Guadalupe County, near San Antonio. Also, one of the California cases -- a 10-year-old boy -- traveled to Dallas this month.
Swine flu symptoms are mostly involving fever, cough and sore throat, though some of the seven also had vomiting and diarrhea.
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"Our hope is that this will blow over," Goodman said.
NBC medical reporter Robert Bazell said the CDC is very concerned because there is genetic evidence of human and bird components in this virus.
Investigators are checking reports of a massive outbreak of a serious respiratory illness in Mexico City where 900 have fallen ill and 16 have died. All of the capital city's schools were closed Friday as a precaution.
So far, testing indicates some mainstream antiviral medications work against the virus.