When people say airplanes are the safest way to travel, there’s something they’re probably not ­considering—disease. Commercial airlines are very good at getting passengers to their destinations in one piece, but those ­passengers often start coming down with illnesses soon after their flights descend from the skies. 

In one study published in American Journal of Epidemiology, researchers followed up with 54 passengers who had been on a flight with someone who had the flu. The researchers discovered  that 72% of those passengers had fallen ill within 72 hours. 

Another study—this one conducted by researchers at Harvard ­University—determined that the air-traffic shutdown following the September 11 terrorist attacks and subsequent ­decline in air travel actually delayed the start of flu season across the US that year. 

Common colds and flu aren’t the only dangers—sometimes passengers are exposed to potentially deadly germs. In 2003, a flight departed Hong Kong with 120 passengers and crew on board—including one who was infected with the SARS virus. Within days, 20 of those passengers and two flight attendants had the potentially fatal disease. 

Airline passengers spend hours stuck in a small space with fellow travelers, breathing the same air and touching the same surfaces—surfaces that might not get cleaned well or often. And their body’s natural defenses against germs are compromised by the ­environmental conditions on planes. Beyond germs, the shrinking airline seats, when ­combined with lack of movement, can lead to other serious health issues as well. 

Luckily, there are ways to protect yourself, particularly on longer flights…

Stay hydrated. You need moist mucous membranes to create a line of defense against germs that are inhaled. The air in airplane cabins is very dry—­often only about 20% humidity or less. Healthful levels should be closer to 40%. Low humidity dries out the ­mucous membranes in passengers’ noses and throats, limiting their ability to shield them from germs in the air they breathe. Drink four to eight ounces of fluid—preferably water—per hour. Avoid alcoholic beverages that can dehydrate the body. 

Warning: Water is, of course, an excellent beverage for hydration—but consume bottled water, not water from the plane’s bathroom tap, which can be contaminated with germs. 

Turn your air vent on, but don’t point it at your face. In past decades, the standard advice was to leave the air vent above airline seats turned off—it blew air that could contain germs from passengers seated throughout the plane. That risk has been greatly reduced because commercial aircraft now have very effective air-filtration systems. In fact, breathing filtered air blowing from the vent above your seat could reduce your exposure to germs from a sick passenger seated near you. Still, filtration systems are not perfect, and this blowing air could further dry out your mucous membranes, so a prudent compromise is to turn your vent on but not aim its stream of air directly into your face.

Find the healthiest seat. What’s the healthiest seat on the plane? People often ask whether aisle seats or window seats are the better choice from a health perspective. The answer is, it’s a trade-off—aisle seats make it easier to get up and move around, avoiding blood clot risk…but window seats mean that you have fewer passengers seated near you, decreasing the odds that you’ll be near someone who is contagious. 

Also: Politely ask a flight attendant if you can switch seats if someone near you on the plane is repeatedly sneezing or coughing. Passengers within three seats to the front or sides of someone who is contagious—or within one to two seats behind such a person—face much higher infection risks than ­passengers seated elsewhere on the plane. 

Pack a surgical mask in your carry-on bag when you fly. Even if you feel embarrassed, don this mask if you are seated near someone who is coughing or sneezing and you cannot switch seats, perhaps because the flight is full. You can buy a box of 100 online for less than $10. Looking silly beats getting sick. At-risk or elderly passengers should consider wearing a mask even if no one around them appears to be sick. Not all contagious people show obvious signs of an illness. 

Get up and move every hour or two. Stroll up the aisle and back or at least stand and stretch. Spending long hours stuck in a cramped airline seat isn’t just uncomfortable…it can lead to a ­potentially fatal blood-clot condition called deep vein thrombosis—even in travelers who have no history of circulatory problems. If it isn’t possible to move around because the “fasten seat belts” sign remains on, at least flex and stretch the muscles of your legs, feet and toes and do some shoulder rolls. Note: Qantas, which plans to start running the world’s longest flight—from Sydney to London in 19 hours—has created a video to help passengers benefit from easy-to-do inflight exercises. Go to YouTu.be/Gv7enzI7Yq8

Wear eyeglasses rather than contact lenses. The low humidity on airplanes can dry out your eyes, creating discomfort for contact lens wearers. If you must wear contacts on a flight, ­apply eyedrops periodically to keep your eyes moist. 

Bring hand sanitizer and/or sanitizing wipes. To prevent the transmission of germs, use sanitizer on your hands during flights immediately before eating, drinking, removing contact lenses or doing anything else that involves touching your eyes or face on flights. Choose a product that contains at least 60% alcohol, and if you opt for hand sanitizer, use a quarter-size drop for each hand cleaning—most people don’t use enough. Also wipe down the seat-back tray table before using it—these are rarely cleaned by airlines and could be contaminated with germs from passengers on prior flights. 

Washing your hands thoroughly with soap and water ordinarily is as effective as using hand sanitizers or sanitizing wipes, but hand washing can be problematic on commercial aircraft. It sometimes isn’t possible to get to the bathroom to wash your hands immediately before eating or drinking—the food cart might block your path…the handles of airplane bathroom doors and soap dispensers are themselves often contaminated with germs…and sometimes there are germs in planes’ tap-­water reservoirs. Always wash your hands thoroughly as soon as you deplane.