Boost your immune system and avoid the dirtiest areas on airplanes to stay healthy this holiday season.

Many people swear they get sick after every winter break.

It’s often not clear whether the cause is the close quarters of an airplane, indulging in more celebratory wine than usual, or the stress of fitting in parties, family time, and long-distance flights.

It might not be one specific thing, but a perfect storm of holiday demands, according to science.

More than 51 million passengers are expected to take flights on U.S. airlines this holiday season.

That’s a lot of people sharing spaces and germs.

Exposure to more people, bacteria, and viruses in an enclosed environment is a key reason we’re so likely to get sick on a flight.

One study found it’s more than 100 percent more likely for someone to catch a cold on a plane than in daily life.

Passengers may be even more susceptible if their immune systems are weakened during the weeks leading up to holiday travel.

If you’re flying over the holidays, start protecting yourself now with science-backed advice and recommendations from three doctors.

Between working long days to fit in projects before winter vacation and celebrating the season with spiked eggnog and mulled wine, people often push themselves before they fly home for the holidays.

Then their bodies go kaput.

Stress, lack of sleep, and dehydration affect the immune system.

“Stress definitely plays a role in illnesses such as colds and the flu. That’s because cortisol, a stress hormone, can affect the body’s immune response, which you need to fight off infection,” explained Dr. Holly Kim, an infectious disease specialist at the Kaiser Permanente West Los Angeles Medical Center.

“Our ability to fight colds and the flu is also compromised when we are exhausted and our immune system becomes weakened. That’s why adequate sleep is so important,” Kim told Healthline.

A 2015 study showed how critical sleep is when it comes to staying health.

Participants who slept less than six hours a night for one week were more than four times more likely to catch a cold than people who slept more than seven hours.

The weeks before a flight, things such as exercise, meditation, soothing baths, and lavender essential oil are important — whatever helps you de-stress and get a full night’s sleep.

Imbibing more than usual on festive alcoholic drinks — and then caffeine to get through the next day — causes dehydration. It also increases cortisol.

Coupled with being in a pressurized compartment, which also induces dehydration, and a long-distance flight could leave a passenger’s body fully parched.

So even if you’re toasting throughout the festive season, drink a lot of water.

“Hydrating with water before the flight prepares your body for the event,” according to Dr. Yvette McQueen, an emergency physician and travel doctor.

Vitamins and supplements, like EmergenC, are probably not helpful.

Vitamin C is necessary for the immune system to function properly, but you’re likely getting enough from fruits and vegetables, said Dr. Daniel Caplivski, associate professor of infectious diseases at the Icahn School of Medicine at Mount Sinai in New York.

If your seasonal diet mainly consists of cocktails and gingerbread cookies, throw in some greens, broccoli, bell peppers, and strawberries.

Passengers sitting in the aisle are in the line of fire for germ-covered hands.

People walking up and down the plane — many coming back from the bathroom — have a tendency to grab onto the top of the aisle seat for support.

That exposes people in those seats to more people and grosser germs.

In 2008, members of a tour group started experiencing symptoms of norovirus, a nasty stomach virus that causes uncontrollable diarrhea and vomiting. It was so bad that the flight from Boston to Los Angeles had to make an emergency landing.

The Centers for Disease Control and Prevention (CDC) followed up with passengers on the flight to see who had contracted norovirus.

They found that the people sitting in the aisle seats were much more likely to have caught the virus. They also found no link between getting sick and using the bathroom.

When the drink cart comes down the aisle, fliers should be cautious about ordering anything that could contain plane water: ice, coffee, tea, or tap water.

The water tanks on planes are rarely cleaned and are “conducive for microbial growth,” according to a study on aircraft water quality.

The Environmental Protection Agency (EPA) findings haven’t been more comforting. When the EPA tested the water supply on U.S. commercial planes in 2012, 12 percent tested positive for coliform bacteria, which usually indicates fecal bacteria like E. coli.

The EPA has been pressuring airlines to clean up their water for decades, but these 2012 numbers only show a slight improvement over past contamination results.

“Consuming only bottled water is a good idea both for airline travel, and in [countries] in many parts of the world where the water could [contain] bacteria or viruses that might be likely to transmit illnesses like traveler’s diarrhea,” Caplivski told Healthline.

On the flight, avoid alcohol, coffee, and soda, which can also be dehydrating, and drink plenty of bottled water.

Travelers don’t have to overdo it on the water, but should aim for drinking just enough so that their urine is light yellow.

Making a few trips to the restroom to pee will also help prevent urinary tract infections (UTIs) and blood clots in the legs.

The dry air on planes can also make it easier for people to catch something, according to Kim.

It’s especially an issue for people who are prone to respiratory tract infections.

“Mucous membranes lining your mouth, nose, and throat protect against illness-causing microbes better when they are moist. The dry air in airplanes can dry out your mucous membranes, making you more susceptible to infection,” Kim explained.

To keep your mucous membranes working, “you can also use a saline nasal spray but be sure that it has no preservatives — just salt and water,” Kim said.

“Another option is to apply an aloe vera gel or saline gel to the inside of your nostrils with a clean tip applicator,” she added.

“Avoid using antibiotic ointments because we want to be careful about overusing antibiotics. Plus, they won’t help protect against colds and the flu, which are caused by viruses, not bacteria.”

Before takeoff, be sure to wipe down your seat, tray, seat buckles, and seat pocket with a disinfectant wipe.

Those are some of the dirtiest places on airplanes, according to several studies.

“Viruses are spread on objects like doorknobs, handles, and armrests, and they can live eight to 12 hours on objects. After someone with a virus has touched the object, we touch it and then may touch our face, nose, or eyes,” McQueen pointed out.

In addition, plane tray tables have more than eight times the number of bacteria than the toilet flush buttons in plane bathrooms.

In 2007, 60 percent of tray tables tested positive for the deadly super bug Methicillin-resistant Staphylococcus Aureus (MRSA), compared to 6 percent of public restrooms.

Other researchers have found cold and flu viruses and norovirus on plane tray tables.

“Airplanes are crowded places. You can easily catch a cold or the flu if you sit next to someone who is sick or about to become sick. You are likely to touch surfaces, such as the tray and seat buckles, that have been touched by many people,” Kim explained.

“Planes may contain an accumulation of microbes from a lot of people — from the flight you are on, as well as ones from earlier in the day or even week,” she added.

It’s not just the people on your current flight that you might catch something from.

It may be one of the other 51 million flyers this holiday heading to a different destination.