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.