Most viruses don't spread easily on airplanes because of how the air circulates and is filtered, the Centers for Disease Control and Prevention says.
Modern commercial jets recirculate 10-50% of the air in the cabin, mixed with outside air.
"The recirculated air passes through a series of filters 20--30 times per hour," the CDC says.
"Furthermore, air generally circulates in defined areas within the aircraft, thus limiting the radius of distribution of pathogens spread by small-particle aerosols. As a result, the cabin air environment is not conducive to the spread of most infectious diseases."
Still, try to avoid contact with anyone sneezing or coughing. And if you're feeling sick, cover your entire mouth and nose with the inside of your elbow when you cough or sneeze.