NYC mayor says 28 swine flu cases from 1 school

"There are no other clusters evident in New York City," Bloomberg said. "We have seen the kind of flu that does not seem to grow and in a few days, the symptoms seem to be going away."

The city's toll rose from eight confirmed cases on Sunday, all involving students at the private St. Francis Preparatory School in Queens.

More than 100 St. Francis students have complained of flu-like symptoms; city officials said they were still evaluating the students, and didn't confirm whether they all had been tested for swine flu.

Nearly all of the students are feeling better, and the one or two who aren't are feeling the same, Health Commissioner Thomas Frieden said.

"We wouldn't be surprised if we saw other cases" elsewhere in the city," Frieden said, but said the city's survey of local hospitals over the weekend hasn't yet found any new cases.

St. Francis officials learned that something was wrong there on Thursday when students started lining up at the nurse's office complaining of fever, nausea, sore throats and achy bones.

Some of the infected students said they had recently returned from a spring break trip to Mexico. Frieden said most of the students who had become ill had not traveled recently, meaning most had passed on the infection within their community.

Bloomberg said the St. Francis infection doesn't appear to match the flu strain in Mexico.

"So far we have no evidence that is consistent with the evidence in Mexico," he said. "But ... their definition of serious illness may be different."

The latest New York City figures brought the number of confirmed swine flu cases in the United States to 40.

However, most of those sickened in the U.S. have recovered or are recovering. That's a stark difference from the deadly outbreak in Mexico that authorities can't yet explain.

The U.S. government has declared a public health emergency to respond to the outbreak, which also has sickened people in New York, Kansas, California and Texas. Health officials in Michigan said they have one suspected case. Many of them had recently visited Mexico.

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