If Hillary Clinton looses the election it will likely be because of her perceived health issues. She made a choice to go to the 9/11 memorial event despite the fact that she was sick and the subsequent complications could change some people's perception of her. However most of the people complaining already didn't like her.
While many oppositions will try to use her health issues against her, especially the likes of Donald Trump; I strongly doubt that this will cost her the election. The election has pretty much been put on a determined path by now. Not much can alter it to a different direction.
Hillary Clinton's health issue will prove to be a non-factor. There are nearly two months left in the election, providing plenty of time for her to recuperate and show she is healthy on the campaign stump. Her pneumonia will heal within a relatively short period of time, and the public will see this is as a non-issue.
Hilary Clinton's health will probably not cost her an election, however, with being in one of, if not THE most stressful job in the world, she is really putting herself at risk. She is 68 years old, and its not in perfect health. The only past presidents around her age were Harrison, Reagan, and Buchanan. But as far as I know they were in great health. Her starting at an advanced age, with existing health issues, may backfire at a crucial time.