Serious eww. WSJ via Around the World blog:
Air that is recirculated throughout the cabin is most often blamed. But studies have shown that high-efficiency particulate air (HEPA) filters on most jets today can capture 99.97% of bacterial and virus-carrying particles. That said, when air circulation is shut down, which sometimes happens during long waits on the ground or for short periods when passengers are boarding or exiting, infections can spread like wildfire.And don't even think of touching the tray table or seatback pocket (just think what people put in that pocket). Ycch.
One well-known study in 1979 found that when a plane sat three hours with its engines off and no air circulating, 72% of the 54 people on board got sick within two days. The flu strain they had was traced to one passenger. For that reason, the Federal Aviation Administration issued an advisory in 2003 to airlines saying that passengers should be removed from planes within 30 minutes if there's no air circulation, but compliance isn't mandatory.
Much of the danger comes from the mouths, noses and hands of passengers sitting nearby. The hot zone for exposure is generally two seats beside, in front of and behind you, according to a study in July in the journal Emerging Infectious Diseases, published by the U.S. Centers for Disease Control and Prevention.