I have to guess that the busy travel season of Thanksgiving, Christmas, and New Year plays a part as well. These are peak crowd-gathering holidays, at least in

I have to guess that the busy travel season of Thanksgiving, Christmas, and New Year plays a part as well. These are peak crowd-gathering holidays, at least in the U.S. Lots of visiting relatives, lots of transiting through crowded airports, lots of cramped airliner cabins and recycled air, etc.