Airlines stress that the cabin air is continually scrubbed of germs as it circulates through the airplane, and that's true. However, the air is also pressurized, which strips it of moisture. This drier-than-desert air dries out your mucous membranes and lowers your resistance to germs around you. If you're sitting within two rows of someone with a cold, the dry, recirculated air makes you more likely to become infected.
Even if you aren't seated near a sick person during your flight, you may still be at risk for infection. Airlines make money by keeping the planes full and flying. There's only a cursory cleaning -- if any cleaning takes place at all -- between flights. Cold and flu viruses can live for hours on surfaces, such as your seat, counters and faucets in the bathroom.