This has been a flu season like few others.

Normally at this time of year, influenza is rampant in the U.S., prompting hundreds of thousands of people to stay home in the dead of winter with fever, aches and pains.

Now, after raging through college campuses and communities last summer and fall, cases of the new H1N1 swine flu virus have dwindled to a trickle, and run-of-the-mill seasonal flu has barely made an appearance. Not one state reported widespread flu illness to the Centers for Disease Control and Prevention for the week ended Feb. 20, the latest data available. The percentage of all doctors' visits by patients with influenza-like symptoms has dropped from a high of 7.8% in late October—the largest peak since the agency began surveillance in 1997—to 1.8% in late February, well below the norm for flu season.

Doctors and flu experts say the lull is unusual. "This is typically the peak of flu," said James Turner, executive director of the University of Virginia's department of student health. He said the Charlottesville, Va., student health center usually sees as many as 130 students a week complaining of flu symptoms this time of year. Recently, no more than three to five students a week have been coming in with fever, cough or other signs of flu, he said.

It is not clear why there is so little flu, particularly swine flu, going around, experts say. "Surely there's a sufficient number of people who haven't been infected or vaccinated," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

Many scientists say the answer probably has to do with how the flu virus progresses. Influenza comes and goes in waves, normally running from October through May. But pandemic viruses—new viruses that emerge and spread quickly around the globe—often move to a different rhythm, and can reach their busiest stage at unusual times like summer and early fall, although the reason for this isn't understood. Flu has peaked in late February or early March in 20 of the past 26 flu seasons, said Lyn Finelli, the CDC's chief of flu surveillance and outbreak response. But the latest swine flu wave started in August, and peaked in late October, before waning.

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