Heard some good commentary on this very issue on the FiveThirtyEight podcast.
Essentially, the well of discourse surrounding Hillary's health has been irrevocably poisoned by spurious rumors and conspiracy theories. We can't sensibly talk about Hillary's health without the specter of some outlandish claim hanging over it. You know what the source of the whole Clinton Seizure/Parkinson's thing was? Fucking InfoWars. The same site that constantly posits that the New World Order is going to destroy America any day now, that lizard people run international banking, and that the government is creating tornadoes to erase the Midwest. Fun stuff, right?
Normally, people would dismiss his claims for the bullshit that they are, but because it was Hillary Clinton (the person whom conservative America considers the final boss of American liberalism), they enthusiastically seized upon it. So now the news media is running with this story as nearly fact, and anything that relates to Clinton's health immediately becomes suspect. Remember that coughing fit a few weeks back? That kind of coughing fit would be brushed off for any normal candidate, but because it was Hillary Clinton, the alt-right weirdos tried their damndest to shoehorn it into their Seizure/Parkinson's/SuperAIDS theory.
So what do you do if you're Hillary Clinton and you develop pneumonia? As I see it, you've got a few options:
A. Get the pneumonia diagnosis, get treatment, and tell people that you're not going to make the 9/11 memorial
B. Get the pneumonia diagnosis, get treatment, and don't tell people that you're not going to make the 9/11 memorial
C. Get the pneumonia diagnosis, get treatment, go to the 9/11 memorial and pray to god that nothing happens.
Both scenarios A and B would cause the conspiracy crowd to shout from the rooftops that Hillary's Zika Seizures are flaring up and that she's literally days away from dying. The headlines on Drudge would read "HILLARY'S LAST DAYS?". It would be an absolute frenzy of bullshit. Clinton gambled on C, and it didn't work, but it was also the only option that had a shot of working, so I agree with her decision to go that way. This incident proves that no matter what Hillary's actual health problems are, her detractors will always find some way to insert their own bizarre irreality into the narrative.