Here's a short but accurate article about air circulation on airliners.
I have flown a lot in my time and rarely, maybe 4 times, gotten sick immediately after and I catch everything. Who knows, too, it could have been from airport bathroom, waiting area, before trip, etc.
Deductively, I have always concluded airplane air not as contaminated as people like to believe.
From NBC in 2010. Nice positive media piece from the swine flu era, and who was prez?...... Odumbo.