U.S. health officials are seeing a surprisingly high number of cases of ordinary, seasonal flu at a time when the flu season typically peters out.

About half of people recently testing positive for the flu have the new swine flu virus, Dr. Daniel Jernigan of the Centers for Disease Control and Prevention in Atlanta said Friday.

The rest have seasonal flu, which is still causing widespread or regional illness in about two dozen states, “something that we would not expect at this time,” he said. “We would be expecting the season to be slowing down or almost completely stopped.”

The higher numbers of seasonal flu cases do not seem to be just because health officials are looking harder this year because of worries about swine flu, Jernigan said.

A network of doctors who track how many patients are coming in with flulike symptoms, plus evidence from school outbreaks and lab testing, points to more flu – not just more reporting, he said.

In the United States, there are now more than 4,700 probable and confirmed cases of swine flu, and 173 hospitalizations and four deaths, Jernigan said. The tally doesn’t include a fifth death that Texas officials said Friday was due to swine flu.

“The H1N1 virus is not going away,” Jernigan said. The virus “appears to be expanding throughout the United States” and poses “an ongoing public health threat,” he said.

Swine flu continues to affect more younger people – those ages 5 to 24 – and CDC is still seeing relatively few cases in older people.

“That may be just a matter of time” until the virus spreads to that population, or it may prove to be a difference in the virus or its effect on various groups, he said.

Officials are still monitoring the situation in Mexico, where the outbreak began.

However, the CDC’s quarantine chief, Dr. Martin Cetron, said the agency was downgrading its warnings about travel to Mexico.

The CDC had urged people to avoid nonessential travel to that country, but that was changed Friday to just a precaution for people at high risk of flu complications.

The fifth U.S. death attributed to swine flu was reported Friday in a 33-year-old Texas man who died May 5 or May 6 after becoming sick a few days earlier. Corpus Christi-Nueces County Health District’s Dr. William Burgin Jr. said the man had medical conditions, including heart problems, that made it tougher for him to fight a viral illness. The victim’s name was not released.

It was the third swine flu death in Texas. Other deaths occurred in Washington and Arizona.

In New York City, three public schools in the borough of Queens were closed after hundreds of children were sent home sick this week, and a city official said Friday that three more schools would be shut down after students developed flu symptoms. At one school, Susan B. Anthony middle school, there were five confirmed cases of swine flu, including a 55-year-old assistant principal hospitalized in critical condition.

Outgoing City Health Commissioner Dr. Thomas Frieden said Friday that the large clusters of apparent swine flu cases at the schools were “a little surprising,” but added: “So far it doesn’t appear to be causing more severe illness than seasonal influenza.”

“We don’t know how far it will spread, how wide it will spread, how long it will spread,” said Frieden, who on Friday was named director of the CDC.

As he spoke, maintenance workers at the schools scrubbed desks, floors and door handles Friday. At one middle school, where 241 children were out with flulike symptoms Thursday, a worker in a mask was seen mopping down the cafeteria.

Addressing criticism that the schools should have been closed sooner, Mayor Michael Bloomberg said, “We have to make decisions on each school individually. … Our children need more time in school, not less.”

New York City’s first swine flu outbreak occurred when hundreds of teenagers at a Roman Catholic high school began falling ill following the return of several students from vacations in Mexico.

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