The flu bug waited to bite this year until spring blossoms were on trees in the San Joaquin Valley, but it's making up for a slow arrival.
Doctors are seeing more achy, coughing, feverish patients, and schools are reporting an uptick in absences because of the flu.
And it's too soon to know whether the worst is over, doctors say.
"We're seeing pretty much epidemic activity," said Dr. Randy Bergen, clinical head of the Northern California Kaiser Permanente influenza vaccine program, which includes Fresno.
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Kaiser judges flu activity based on the percentage of positive influenza tests -- any rate above 30% is considered significant. For the third week in a row, it's been above 30%, Bergen said. This past week, it was 37%.
Doctors at Children's Hospital Central California in Madera County reported 105 confirmed flu cases the first week of March, compared with 17 cases the same week in February.
"The slope keeps going up," said Dr. Jim McCarty, Children's pediatric infectious disease subspecialist.
It could be a couple more weeks before flu season peaks, said Jeff Dimond, spokesman for the federal Centers for Disease Control and Prevention. But he can't be sure. "Flu by definition is totally unpredictable. It's the most vexing virus out there," he said.
In California, flu season typically peaks sometime in February or early March, said Dr. James Watt, chief of the division of communicable disease control at the California Department of Public Health.
Last year was the weird year, he said. The peak happened in the fall. "This year is very much our typical pattern," he said.
And it's not too late to get a flu shot.
The vaccine remains widely available, with more than 160 million doses produced for this season nationwide. Many pharmacies are offering flu shots. The vaccine protects against three strains, including swine flu.
There also is a bright spot. This year's flu season appears milder than what doctors saw last season with the arrival of the H1N1 swine flu strain in 2009.
"We're not seeing ... that degree of severity or intensity," McCarty said.
There were 22 influenza-associated children's deaths in the 2009-10 season in California. All of those deaths had been reported by this time last year. So far this year, there has been one flu-related death reported, a child in Ventura County.
This year, swine is one of three influenza strains making people sick.
Dean Gregory, the head varsity softball coach at Buchanan High School, spent seven days at Clovis Community Medical Center in January with swine flu and pneumonia. "I got out of the hospital the day before the Super Bowl," he said.
Schools are reporting children out sick with the flu -- just not to the degree they saw last season.
Absence rates from a handful of schools that make reports to the Fresno County Department of Public Health have been under 2%, said David Luchini, manager of the department's communicable disease division. None of the schools are reporting rates of 5% or 6% as were seen last season, he said.
But flu season isn't over.
At Community Medical Providers on Herndon Avenue in northeast Fresno, patients -- young and old -- began trickling in last month with flu-like symptoms, said Walene Herzog, office manager. "We're just starting to get hit with it," she said.