Six years ago, Alissa Dorman boarded a plane from her home in the Pacific Northwest for Arizona. It was her family’s first destination vacation and the first flight for her son, who was 3 at the time.

Three days later, Dorman was hooked up to an IV in an Arizona hospital, fighting a gastrointestinal disease. Though she can’t be sure where she picked up the illness, she said, “I’m guessing it was probably from an airplane.”

The five hours in the emergency room didn’t ruin her trip — or curb her enthusiasm for traveling. Since then, Dorman’s made several trips to Maui, as well as Orlando, Arizona and Mexico. But the experience dramatically altered how the young mother travels.

“It always seems like we travel in the peak flu season — before spring break or the middle of the fall,” Dorman said. “My fear is that we’ll get sick right before or during the trip.” Now her carry-on is packed with antibacterial wipes and gels, the overhead fan stays off and she makes frequent trips to the bathroom to wash hands (being careful to open and close the lavatory door with a towel over her hand). During the week before the trip, Dorman takes vitamins. She hasn’t been sick while traveling since.

Preventing sicknesses while traveling is an industry of its own. Tablets that promise to curtail absorption of airborne pathogens, masks that aim to prevent airborne sickness, even clip-on filters for seat vents are marketed to passengers.

But are such measures necessary while flying?

The Five-second Rule

Quick! Pick up that pretzel that just toppled out of the snack pack onto your airplane seat tray. As long as you pop it into your mouth within five seconds, it’s still safe to eat, according to commonly held belief.

Not so, warned Dr. Paul Dawson, Food Science and Human Nutrition professor with Clemson University. Dawson’s team published a study three years ago in the Journal of Applied Microbiology that debunked the “five-second rule” — the idea that food dropped on the floor or other surfaces is safe to eat so long as it’s picked up within five seconds.

Dawson’s study found bacteria makes the leap from tabletops and floors to food well within that short time frame.

San Diego State University professor Dr. Scott Kelley’s January research supports Dawson’s study. Kelley’s team, in a study funded by The Clorox Company, dropped three baby carrots in a sink, on a tile floor, carpet and tabletop in two homes. Germs attached to clean carrots within five seconds. Similarly, germs grabbed onto a child’s sippy cup tested in four homes by dropping the cups onto high chairs, tile floors, carpets, countertops and tables within five seconds.

Despite these test results, 65 percent of parents did admit to following the five-second rule in their home in a 2009 survey of 500 U.S. families. This is even on high-chair trays, which were found to be sources of high levels of potentially illness-causing germs.

The good news is that other 2007 studies showed that an airplane seat tray isn’t as germy as many believe.

The National Institute for Occupational Safety and Health (a division of the Centers for Disease Control and Prevention) used a cotton wool-tipped stick to collect samples from armrests and tray tables on 12 randomly selected Boeing 767 flights. Each selected flight traveled between four-and-a -half to six-and-a-half hours and ranged between 67 and 100 percent occupancy. The team also collected samples from the armrests of chairs and end tables in the guest area prior to boarding.

The bacteria found were common to skin and mucus membranes, the majority of which are harmless. According to the study, “The bacterial concentrations we observed should not cause adverse health effects for healthy individuals traveling on an aircraft.”

A separate study evaluated fungal populations on wide-body commercial passenger aircraft and in airport terminals using similar techniques. The fungal populations inside the plane had “lower mean concentrations than those observed in typical office buildings,” said Dr. Lauralynn Taylor McKernan, senior environmental health officer with the Centers for Disease Control and Prevention, in an email.

United Airlines spokeswoman Sarah Massier said the airline frequently cleans cabins between domestic flights and steam-cleans the wide-body cabins.

So that pretzel probably does have a higher bacterial and fungal load than prior to landing on the tray, but if that doesn’t make you queasy, go ahead and eat it. You aren’t likely to get sick from it.

Take a Deep Breath

Flight attendants claimed a higher-than-average number of respiratory illnesses in the December 2003 study by the NIOSH entitled “Prevalence of Respiratory Symptoms Among Female Flight Attendants and Teachers.”

But that doesn’t necessarily mean that airplanes are the breeding ground for colds and flu that over-the-counter vitamin companies might want travelers to believe. “We did not measure rates of infection, but simply looked at self-reported symptoms in flight attendants compared to a non-flying occupational group,” said Dr. Elizabeth A. Whelan with NIOSH, in an email.

Humidity is lower on board planes during flights, so the CDC recommends passengers stay hydrated to keep nasal passages moist and able to filter germs and to flush toxins.

“When I travel frequently, I try to drink plenty of water to stay hydrated and wash my hands frequently,” Dr. McKernan said.

The 2007 studies did show a spike in airborne bacteria and fungi in the air during boarding and deplaning of flights they sampled.

Researchers hypothesize that passengers carry fungi and bacteria from outside when they board. Bacterial levels were higher in the front of the cabin during boarding, likely because passengers lined up in the front of coach class to board back to front.

“To minimize the time on board during boarding, passengers might consider boarding later, if concerned,” Dr. McKernan said.

The NIOSH research showed that on the Boeing 767 flights sampled, the filtration system operated efficiently when two air packs and a 50 percent recirculation rate were used during operations.

Boeing aircraft keep continuous airflow. About half the air leaves the cabin through outflow valves in the lower lobe. The other half is drawn by fans through High Efficiency Particulate Air (HEPA) filters under the cabin floor, according to the Boeing Company website.

The filtered air then mixes with outside air coming in from engine compressors. “The HEPA filters are very effective at trapping microscopic particles such as bacteria and viruses, and can provide essentially particle-free air in the recirculation system,” according to the website.

“Cabins are not like hazardous sites,” said Steve Howards, executive director with Clean Airports Partnership. “They’re significant sources of air pollution, but I would not worry about health issues if I were a traveler, which I am.”

Howards travels about 200,000 miles a year by plane from his Colorado home to consult with airlines and airports on ways to improve their efficiency and environmentalism. He said he’s much more concerned with larger-scope pollution issues than breathing in germs while on board. “Since it is recycled air, the best thing you can do is wash your hands and get in healthy,” Howards said.

To Sit or Not to Sit

For most global travelers, a pit stop in the airplane lavatory is inevitable. Unfortunately, the germs inside might be, too. “This makes sense, given the purpose of the lavatory and the fact that it is the most commonly visited area [of the airplane],” according to the summer 2004 study, “Molecular Survey of Aeroplane Bacterial Contamination,” by San Diego State University.

The university team studied four airplanes. The research included wiping down the entire surface of toilet handles, sink handles and door knobs. According to the study, “The substantial diversity of human-associated bacterial species found on numerous lavatory surfaces on every flight tested, including a paper towel, suggests that lavatories represent the biggest potential hazard for passengers and crew.”

Among the organisms detected were those typically found orally in humans, and were likely spread from passengers coughing or blowing their noses. “Our results indicate that aeroplanes have the potential to spread an enormous diversity of bacterial species among passengers and destinations,” said the study.

This might sound alarming, but keep in mind that the study also found that the organisms detected were “not especially threatening to the general public,” and in order to not frighten passengers, researchers did not don sterile latex gloves while collecting samples (although they did wash their hands).

Adopting Dorman’s technique might help worried flyers avoid bringing bathroom germs along to destinations.

When I use the bathroom on longer trips, I always wash my hands, open the door with a towel and then use the gel when I get back to the seat,” she said. “The ironic thing is, when I get to my destination, I’m much more at ease and comfortable. Not that I don’t wash my hands, but not nearly as much as prior to the trip.”