The nasal and paranasal cavities. The source of joy for so many people around the world, and yet another proof of the intelligent design ad hoc nature of evolution.
I always have had hayfever throughout the year. Once when my GP asked my at what time of year I have the most problems my reply was “Well, it usually is the worst from January til December.”. That was a rare occasion, because he does not laugh often.
But despite constantly runny nose alternately either due to cold or due to allergy, I never had serious problems with sinuses. Until last two years. Maybe the subtle changes in bone structures due to aging came over some tipping point.
What I remember from school about this predicament is that our upright posture is responsible for most of it. The sinuses evolved in quadrupeds and they use gravity for draining the phlegm. Evolution has tried to keep up with our evolving of upright posture and flat face, but did not manage it well enough. Thus our species is blessed with the ability to flip from just annoyingly runy nose to headsplitting ache overnight.
As I said at the beginning. Oh the joy.