Um, not sure how this was not known..

From the article:

“We found that flu cases contaminated the air around them with infectious virus just by breathing, without coughing or sneezing,” said Dr. Donald Milton, who’s been studying flu transmission at the University of Maryland’s school of public health.

“Even if you are not coughing, you can still infect other people,” he added.

“Many people shedding virus into the air are shedding real, infectious virus.”

Huh. So you could be sitting next to someone with the flu who doesn’t sneeze or cough and still catch it.