Worried about your immune system?  There may be no better way to test it out than flying on an airplane, according to an article on cnn.com.

A microbiologist was sent by the website Travelmath.com to take bacterial samples from five U.S airports and four flights on two major carriers to see what public surfaces were the best breeding grounds for germs.  The website did not say which airports or airlines were tested.  A total of 26 samples were taken.

The survey showed the airplane tray table was the dirtiest surface tested, with 2,155 colony-forming units (CFU’s) per square inch.   Airport drinking fountains placed second on the list with 1,240 CFU/sq. in., followed by overhead air vents on airplanes.

Not surprisingly, lavatory flush buttons, seatbelt buckles, and bathroom stall locks, rounded out the list, with those last three at 285 CFU/sq. in. or less.

Speaking about the tray table, Travelmath’s report stated, “Since this could provide bacteria direct transmission to your mouth, a clear takeaway from this is to eliminate any direct contact your food has with the tray table.”

Airplanes and airports have long been thought to be prime areas for bacterial breeding grounds, since they have continuous streams of people and lots of enclosed areas with re-circulated air.

Most of the time there is a short turnaround from deplaning and seating new passengers for the next flight, and that leaves airline staff at a disadvantage when trying to clean up after the previous passengers.

Airport bathrooms are usually cleaned quite regularly during the day, which also may be reflected in the fact that the dirtiest area is onboard the aircraft.

But, Travelmath says you should not avoid flying based on this analysis.  The website didn’t identify the bacteria found, but said all 26 samples tested negative for the presence of fecal coliforms, like E.coli, that could be potentially infectious.