The following was a question sent in to Marilyn Vos Savant of Parade Magazine:
Odvar Holm of Carlsbad, California, writes:
Marilyn: If someone with a cigarette is walking 10 to 15 feet ahead of me, I can smell the smoke. But say the person has the flu instead. Can I catch it from the breath he or she is exhaling?
Cigarette smoke lingers in the air, so you can easily smell it as you walk in the wake of a smoker, even outdoors. But influenza viruses are spread primarily with large-particle droplets that don’t remain suspended in the air. They travel only a few feet and then settle. You would have to be walking close behind a person who coughs or sneezes to catch a virus he or she is carrying.
Small-particle transmission (via the residue of evaporated droplets) may be possible because these particles might remain airborne, but data on this is limited.