PHOENIX -- Many people think they get sick after flying in airplanes, but is it their imagination or are germs really worse on planes?

A few months ago, a flight attendants' union put out a warning after two mumps patients traveled on a number of flights, possibly spreading the disease.

5 Investigates took to the air to find out just how dirty planes really are.

Four airlines were tested, and only one passed.

5 Investigates took flights on Delta, Southwest, US Airways and Mesa Airlines, and swabbed areas that most people come into contact with: the seat, seatbelt, armrest, tray table and attendant call button.

The samples were packed in ice and shipped to the University of Arizona for testing.

"Overall, we found that the samples that were collected on the airplanes were heavily contaminated with general bacteria," said microbiologist Kelly Reynolds.

Of course, there are bacteria in every public place. Anything below 1,000 bacteria is considered normal.

But most of the areas tested by 5 Investigates registered off the charts.

"One site in particular was the call button, so that tiny little button you press to call the stewardess or the steward -- we found over 100,000 bacteria," Reynolds said.

That's more than 100 times the expected amount.

"Oh, that is so gross. That is so gross," said frequent flyer Andie Abkarian.

Abkarian flies enough to know that she needs to carry hand sanitizer on board.

"It always crosses my mind: what could be on that button or on that armrest?" she said.

She's not the only one.

"I'm constantly washing my hands, and that's all we can really do," said a flight attendant who's worked with two different airlines during the last 10 years.

"Oh, you see food on the floor," the flight attendant said. "I've seen people put diapers in the seatback."

There is no regulation when it comes to planes being sanitized from top to bottom, which turns them into traveling germ factories.

"Airplanes could serve as sort of a transition route between different areas geographically for spreading germs," Reynolds said.

US Airways had the lowest germ count on a single flight, according to the 5 Investigates test.

In a faxed statement, the airline said, "We … clean our aircraft -- including overhead bins, tray tables, armrests and lavatories -- with disinfectant after every flight."

"Southwest does what we call a maximum effort cleaning," said Scott Camacho, of Southwest Airlines.

Southwest Airlines came in second in the 5 Investigates.

Camacho represented the only airline that would go on camera to address the 5 Investigates' results.

"The aircraft is scrubbed down from the ceiling to the carpet every 30 days," Camacho said.

Delta Air Lines took third place in the 5 Investigates test with extremely high bacteria levels.

The worst was Mesa Airlines. That flight maxed out the test at more than 100,000 bacteria for every swab sample.

Neither Delta nor Mesa would comment on the 5 Investigates findings.

Airline customers are left to wonder what other germs may be flying the friendly skies.

"Knowing that, I probably would be much more careful about pouring out a little snack," Abkarian said.

The good news is that none of the tests came back positive for E. coli or Staphylococcus aureus, but if a surface is covered with high levels of bacteria, people are much more likely to pick up a virus.

The best method of protection is to bring a bottle of hand sanitizer on board -- regulations require that the bottle be no larger than 3.4 ounces -- or sanitizing wipes to clean off surrounding surfaces.

Click here to view the 5 Investigates' test results.