This is a story, I'm sad to say, I had a front row seat to. I was flying from Dallas to, I think, Phoenix and a very large gentleman was sitting on the inside seat while I had the window. I'm like the other poster where takeoff = fast asleep. However 30 minutes into the flight I was awakened to the sounds of horrendous vomiting. The very large gentleman next to me was puking violently into an airline vomit bag (which smelled like peanuts), while facing me. THEN, after his fits of puking are done, there is a smell of s**t that starts to fill the cabin. The poor guy actually puked so hard, he soiled himself.
So by now the flight attendants are aware of it and are helping him. Luckily he has a change of clothes in his carry-on. As he gets up to change I cannot begin to describe the smell, the amount of dry heaves, and coughing that began. While he is changing, the attendants are frantically scrubbing the overhead bins with some flowery stuff to try to cover the smell while another is changing out the seat cushion. The worst part is this poor soul had to sit in his seat for another 1.5 hours, or however long the flight was, with an entire plane full of people who wanted to strangle him.
I must have looked like I witnessed a train wreck when he left for the bathroom because a little old lady behind me tapped me on the shoulder and said, "are you ok dear?"