If you flew anywhere last summer, you know how crazy things got. There were record-setting flight delays and cancellations. Many people were stuck in travel hell for days on end.
Things have settled down quite a bit since then. Still, high travel demand, limited flight availability and winter weather could result in chaotic cancellations and delays during holiday travel. So in addition to the time you spend on the airplane itself, you could find yourself hanging out in airports a lot longer than you would like.
Between the stress caused by potential delays, and being trapped in a huge tub of germs for hours at a time (think of your plane as a giant petri dish), the chance of getting sick within a day or two of reaching your destination is high.