Until now, most people thought you caught the flu after being exposed to droplets from an infected person’s coughs or sneezes, or by touching contaminated surfaces.

But a new study in the Proceedings of the National Academy of Sciences reveals that we may pass the flu to others just by breathing.

The study — which included researchers who are now working at San Jose State University and UC Berkeley — offers new evidence on the importance of the flu’s airborne qualities and how it can easily be transmitted to others. Researchers found large quantities of infectious viruses in the breath exhaled by those suffering from the flu.

“The study findings suggest that keeping surfaces clean, washing our hands all the time, and avoiding people who are coughing does not provide complete protection from getting the flu,” Sheryl Ehrman, who is now dean of the College of Engineering at San Jose State University, said in a statement. “Staying home and out of public spaces could make a difference in the spread of the influenza virus.”

Ehrman said the study was launched at the University of Maryland during the flu season of December 2012 through March 2013. Researchers including Jovan Pantelic, who now works at UC Berkeley, recruited 178 volunteers, mostly students, who had shown flu symptoms within three days of the flu’s onset.

Over those four months, researchers captured and characterized the flu virus in 142 of the volunteers with confirmed cases of the flu while they breathed naturally, talked, coughed or sneezed.

The researchers then assessed the severity of naturally occurring flu aerosols — tiny droplets that stay suspended in the air for a long time.

The study’s participants provided 218 swabs from their nasopharynx, the upper part of the throat that lies just behind the nose. They also provided 218 samples — over a period of 30 minutes — of exhaled breath, spontaneous coughing, and sneezing on the first, second, and third days after the onset of flu symptoms.

The analysis of the infectious virus recovered from these samples showed that a significant number of flu patients routinely shed an infectious virus into tiny aerosol particles that can be transmitted through the air.

Surprisingly, the study suggested that coughing or sneezing was not necessary to be infectious.

“We found that flu cases contaminated the air around them with infectious virus just by breathing, without coughing or sneezing,” Dr. Donald Milton, professor of environmental health in the University of Maryland School of Public Health, said in a statement.

“People with flu generate infectious aerosols even when they are not coughing, and especially during the first days of illness,” he said. “So when someone is coming down with influenza, they should go home and not remain in the workplace and infect others.”

The researchers believe that their findings could be used to improve mathematical models about the risk of airborne flu transmission from people with flu symptoms, and may help control and reduce the impact of influenza epidemics and pandemics.

Improvements also could be made to ventilation systems to reduce transmission risk in offices, school classrooms and subway cars, for example, the study said.

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Next up: Californians brace for the ‘twindemic’ For now, the researchers — and public health experts — say everyone should heed the advice to stay home, if possible, when they’re starting to come down with the flu to prevent the virus from spreading.

And while getting a flu vaccine isn’t a guarantee that you won’t get the flu, experts say it provides some protection and helps reduce the chances that you’ll get seriously ill from the flu.