I agree that this is total quackery.

There is only one way that you can catch a cold: a cold virus living in one person gets transmitted to you.

On a plane, there are basically two ways this happens:

1) airborne transmission: somebody sneezes and you inhale the droplets. this is actually not that common. you need somebody to sneeze or cough droplets into the air and you need to inhale those droplets.

2) droplet contact. somebody sneezes into their hands, touches the tray table or shakes your hand, you touch the tray table and then pick your nose, pick your teeth, or rub your eyes. the virus enters your system via your mucous membranes.

It's hard to prevent #1, but there are some things you can do. what I do is that if I know that somebody has just sneezed or coughed, i put my face away and breathe air away from the "blast" zone. or I hold my breath until the air has dissipated (turn on the vent full blast to dissipate the air). a mask (unlike many of those in Asian countries believe) is essentially useless you use an air-tight mask (such as those for tuberculosis). Regular hospital/surgical masks are completely useless. They are only useful if you put the mask on the sick person, not the healthy person.

#2 is much easier to control: wash your hands. If you haven't washed your hands, don't put your finger in your mouth, nose, or eyes. Don't pick up a sandwich or cookie with your bare hands. use a napkin or the wrapper to pick up the food, and never touch the food with your bare hands. A cold virus living on your hands won't give you the cold; however, once it gets into your mucous membranes, that's when you're toast. Don't ever let the germs on your hands enter your mucous membranes (this includes picking your nose, so wash your hands before you pick or rub your nose).

In reality, the anwer is very simple. Wash your hands; it would cut down on a lot of germ spreading.