Although I wish people didn't fall ill on flights I think most would be surprised how often incidences do occur.
I cringe when I hear the tanoy say " is there a DR or nurse on board, we have an ill passenger" I always wait for a few minutes to see if someone else goes forward and I leave them too it unless they need assistance.
Probably like John as a frequent flyer I have actually had a number of passengers to assist and so far this year there has been two, thats my average per year it always happens to me
On both ocassions the passengers were elderly and shouldn't have been flying at all because they were just not fit, one was after surgery and the other was a recent discharge from hospital with a heart complaint 
Just a reminder, if in doubt always check with your Dr and ask if you are fit to fly, sitting in one position for a long time, pressure in the cabin can all cause problems for those who may not be a 100%.