The novel strain of influenza that circulated around the world in 2009 -- H1N1, dubbed the swine flu -- is going viral again, becoming the predominant strain in the flu season this year.
And as in 2009, the strain seems to cause more severe cases in teenagers and young adults.
"It's going to hit young people," said Dr. Greg Dworkin, a pediatric pulmonologist at Danbury Hospital and founder of the Flu Wiki sites that monitor influenza around the world. "They're not going to expect it."
Dworkin said he saw two or three influenza cases in his office last week.
When the numbers reach that level, he said, it's clear that the disease is about to become widespread.
"If people come into my office with flu symptoms, I'm just going to assume they have the flu," Dworkin said.
Dr. Tom Brown, medical director of Doctors Express, a walk-in clinic on Main Street in Danbury, said he started seeming influenza cases in early December. He said he's seen as many as seven flu cases in one day.
"It's here," he said. "People aren't lining up at our door. But it's here."
The CDC's website on influenza shows Connecticut having "regional" influenza -- with pockets of illness -- while Massachusetts, New York and Pennsylvania were classified as in the more severe category with widespread outbreaks. That report was posted on Dec. 21 and had not been updated as of Thursday.
Dworkin said Thursday he's been somewhat surprised at the resurgence of the H1N1 strain. But he also stressed that every influenza season is different.
"It's influenza," he said. "You never know."
Influenza is a viral illness that causes fever, headache, fatigue, a bad cough and pulmonary congestion. Most people recover after a week or two of feeling miserable. But in small children, seniors, or people with other chronic medical conditions, the flu can create serious complications.
Because flu seasons vary, the CDC has said influenza can kill as few as 3,000 people and as many as 49,000 in a normal year. The disease also hospitalizes about 200,000 people a year, the CDC has reported.
Ordinarily, flu causes the most problems with very young children and older adults. That was true in the past few years, with the H3N2 strain predominating, Dworkin said.
But in 2009, a new strain -- H1N1 -- suddenly appeared. It causes more illness in teenagers and young adults than older or younger people.
Because it was a new strain, it took its toll. The CDC estimated in 2012 that worldwide, about 284,000 people died of the H1N1 influenza in 2009 and 2010.
The best weapon in preventing influenza is the flu vaccine. Dworkin said this year's vaccine is a good match for the strains circulating, including H1N1.
Hjung of the CDC said teens and young adults lag behind the rest of the U.S. population in getting a vaccine.
"We vaccinate about 40 to 45 percent of the adult population, he said. "But in the population between 18 and 49 years old, it's only 30 percent."
But if people haven't been vaccinated yet, they should be, Dworkin said.
"It's never too late," he said.