The low humidity causes dryness in the throat and nose membranes, which can add to the risk of infection. Passengers seated in the rear of the aircraft are at greater risk because airflow there is at its lowest. There are several things travelers can do to protect themselves from arriving at destinations infected by a bug encountered during flight:

First of all, says Dr. Michele Ginsberg, chief of community epidemiology for the County of San Diego, travelers who fly often should consider themselves at high risk for flu -- and get a flu shot early in the season.

"Airlines should also provide flu vaccine for their employees," said Ginsberg. "It's the one proactive thing you can do to really make a difference. It won't protect you against all viruses, but at least you'll be safe against the major strain of the season."

When booking a flight, ask for a seat assignment as far away as possible from the back of the plane, advised Dr. Chris Holmes, a San Diego physician who specializes in occupational and travel medicine. "The bulkhead seats up front are a good place to be."

And, of course, once aboard the plane try to avoid sitting near anyone who appears to be ill.

"Asking to move seats is certainly logical," said Ginsberg, "but it may not be an option.

"If someone is sneezing near you, politely ask them to cover their sneezes. They may not like it, but making a request in a very civilized manner is more reasonable than sitting there continuing to be exposed."

Holmes suggests that travelers wear a disposable surgeon's mask, available from drugstores. "It would look goofy, but it would sure help reduce diseases spread by droplet infection," he said.

Ginsberg said wearing a mask would be especially helpful for travelers with chronic ailments such as heart or lung diseases.

"A lot of exposure is hand to mouth, hand to face," Ginsberg added. "On an airplane you're handling things other people have handled. Studies of people playing cards have shown us how hands transmit germs. The best thing you can do is wash your hands very well when you have the opportunity. Before you eat, have a snack or put anything in your mouth, go to wash your hands."

She suggested carrying sanitary hand wipes in carry-on luggage to facilitate frequent hand washing.

And even though the dry air on planes can irritate eyes, travelers should keep their hands out of their eyes, no matter how clean their hands are. "When your eyes are dry, there's an increased likelihood of abrasion," Ginsberg said. "Use artificial tears to keep your eyes moist."

Dry cabin air can enlarge the tiny cracks in nostrils, making it easier for germs to creep in. Many veteran air travelers protect themselves by coating the insides of their nostrils with an ointment such as Vaseline.

"It couldn't hurt," said Holmes. Ginsberg agreed.

But it could hurt to cuddle up with one of the pillows provided on planes. Most airlines don't replace or clean pillows until the end of the day. So if you're using one, you're essentially sharing it with previous passengers -- and their germs.

"An airplane is a little like sick building syndrome." Holmes said. "Dilution is the solution to pollution. If the windows are open and there's a flow of air, you're washing out the pollutants. When you close all the windows, you're just recirculating the pollution. That's what's happening on an airplane. Not to mention all the sneezing, coughing and hacking of passengers that's added to the mix."

Any good news?

"Since airlines banned smoking, you don't have smoke particles acting as viral transport agents," he said. "That's helped."

Meantime, surgeons' masks may become the hottest new travel accessory.