I was once at a Prom concert in London and the old man standing in front of me with a young boy was clearly incontinent, there were nauseating wafts of fresh micturation dispersed with galling regularity, one might have kept a watch to his bladder. Now imagine *that* on a plane.
It's not a problem I've noticed like you do on buses and trains, though I once sat next to an overweight American in shorts on a domestic US flight in summer who was eating from a bag full of cakes and gave off a horrible pasty smell.
But the last couple of times I've flown I've cycled to the airport with just a sleeping bag and change of clothes, the first time I managed to get a shower from the sprinklers on the golf course where I was bivvying but the second time not, I do hope that wasn't noticeable on the flight next morning, having done 100 miles to get there the evening before.