You don't need anyone to tell you how crammed jet planes are these days. As airlines try to stay competitive, they jam more and more of us closer together leaving our health at the mercy of our fellow passengers. In fact, one recent study found that when 1,000 people traveled between San Francisco and Denver, 20% of them came down with a respiratory illness within the first two weeks after flying. In a famous example, one coughing passenger on a flight from Hong Kong to Beijing infected 22 other people on board with the deadly SARS virus. Five of them died.
One of the reasons planes are such good incubators for illness is that their ventilation systems recirculate air from side to side. That means when the man in the window seat across the aisle from you coughs, his germs fly right onto you. So you don't have to be sitting next to a sick person to be exposed. To make you more vulnerable, the cabin pressure inside airplanes dries out mucous membranes leaving them more susceptible to germs.