Each cough, sneeze or conversation puts more flu virus into already contaminated air.

Across town, doctors at Rush University Medical Center have seen 203 flu patients since November 5, compared with 119 patients for the entire flu season last year.

“We are coming to the point where we are running out of testing supplies,” said Dr. Ed Ward, an expert in emergency and internal medicine at Rush, a teaching hospital.

Similar scenes are being played out in emergency departments across the country as the United States grapples with the earliest flu season in a decade.

“The emergency rooms are quite full and it’s clear that the annual flu epidemic is in full swing,” said Dr. Brian Currie, medical director for research at Montefiore Medical Center in New York.

According to the U.S. Centers for Disease Control and Prevention, the proportion of people visiting their doctor for a flu-like illness has climbed from 2.8 percent to 5.6 percent in the last four weeks. That compares with 2.2 percent during last year’s mild flu season and a peak of 7.7 percent during the 2009 H1N1 flu pandemic.

Dr. Daniel Lucey, who tracks global flu activity at Georgetown University Medical Center in Washington, said people naturally reflect on the past year’s flu season as a measure for the severity of flu, making this season appear all the worse by comparison.

Even so, he said, “it’s an objective fact that flu viruses are circulating earlier and more widely this year than most years.”

Lucey and others say it’s not clear why the flu arrived earlier than normal in the United States this year.

Although each season is unique, flu activity generally starts to pick up in December, peaks sometime in January or February and peters out by late March or early April.

One risk of an early flu season is that it arrives before people have had a chance to get vaccinated.

That may be especially true following last year’s very late and mild flu season, which may have led some to think they had more time. Experts, however, say it is not too late to get a flu shot.

“We strongly encourage people to get vaccinated, and we’d like them to do that as soon as possible,” said Dr. Michael Jhung, a CDC flu expert.