ATLANTA - US health officials are seeing a surprisingly high number of cases of ordinary, seasonal flu at a time when the flu season typically peters out, and about half of the people testing positive for flu have the new swine flu virus.

Dr Daniel Jernigan of the federal Centers for Disease Control and Prevention made the disclosure on Friday in discussing the spread of the virus.

'Those who don't have swine flu have seasonal flu, which is still causing widespread or regional illness in about two dozen states and is 'something that we would not expect at this time,' Dr Jernigan 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.

In another large US outbreak of swine flu, three public schools in New York City were closed on Thursday, and the city's health commissioner, Dr. Thomas Frieden, said Friday that the large clusters of cases were 'a little surprising.' New York City officials said they weren't sure why the virus was spreading so rapidly.

In the United States, there are now more than 4,700 probable and confirmed cases of swine flu, and 173 hospitaliations and four deaths, the CDC's Jernigan said. The tally doesn't include a fifth death that Texas officials said Friday was due to swine flu. Texas health officials said the victim was a 33-year-old Corpus Christi man who had heart problems.

'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.

Dr Jernigan said it 'may be just a matter of time' before older and younger groups are involved. -- AP