Modern day aviation gets a bad rap when it comes to illnesses associated with commercial flight. The truth is, the air in airplane cabins is actually filtered very well every 3 to 5 minutes with HEPA filters. The primary reason you may get ill is because you are flying in close quarters with a crowd of people, and several of them likely have an infection like the common cold or flu. Germs spread through the air before it gets circulated, resulting in exposure. In reality, the nozzles blowing air above your seat may actually mitigate the effects of germs from those around you who may be coughing or sneezing.