Studies show that air travelers suffer hight rates of disease infections than people who move about on the ground. (Although I don't imagine that buses are that germ free either.) One study showed a 20 percent increase among flyers to catch colds. Cabin air-filters catch 99 percent of bacterial and virus carrying particles, but when the plane is on the ground before take-off and after landing, the air circulation system is turned off. That's when sickness spreads like wildfire.
Scott McCarney, in a "Wall Street Journal," wrote: "A number of factors increase the odds of bringing home a souvenir cough and runny nose. For one, the environment at 30,000 feet enables easier spread of disease. [Much of the danger comes from sick passengers sitting nearby. Air in planes is extremely dry, and viruses tend to thrive in low-humidity conditions. When mucous membranes dry out, they are far less effective at blocking infection. High altitudes can tire the body, and fatigue plays a role in making people more susceptible to catching colds, too. Also, viruses and bacteria can live for hours on some surfaces--some viral particles have been found to be active up to a day in certain places. Tray tables can be contaminated, and seat back pockets, which get stuffed with used tissues, soiled napkins and trash, can be particularly skuzzy. It's also not difficult to know why germs are lurking in an airline's pillows and blankets." (I guess it's kind of ironic that the great germaphobe, Howard Hughes, was a commercial aviation pioneer.)
As germ factories, airplanes sound almost as bad as hospitals. Almost as bad because when most people go to the hospital they are already sick and vulnerable to infection. But this could also apply, I guess, to people who fly every day.