If health woes force Hillary Clinton out of the race, what happens depends on when it happens.
The DNC can fill the ticket prior to the election, although with early voting, many would have voted for someone else and the logistics of changing ballots is an interesting twist.
If a candidate won the election, but was incapacitated before the electors voted in December, those electors would be freed to vote for who they wanted, pending state law restrictions. That would mean a Joe Biden.
After the electors choose a President and VP, the normal succession would occur.
Remember some weeks back when the wagging tongues were speculating what would happen if Trump dropped out.
It doesn't take long to start picking the carcass.