I've had pneumonia three times. Though the symptoms below represent my personal experience, I'm fairly certain that they are reasonably typical of most pneumonia patients. The middle time of those three, about four years ago, was the least serious of the three, and the diagnosis was almost identical to Hillary Clinton's: a small spot in one lung.
And even with only a small spot in one lung, I was sick. Very sick. Fever. Strong and painful deep cough. Hard to breathe. Very weak. My doctor, who usually is very lenient about things, was unequivocal: Bed rest for five days (that time - the next episode it was for a week). And after all that it was still several weeks before I was really back to myself.
The point is: Hillary doesn't act like someone with pneumonia. A pneumonia patient diagnosed just two days before wouldn't have been able to even attend that 9/11 memorial. She wouldn't have been able to walk out from her daughter's apartment, waving and smiling; and out of regard for the possibilities of contagion, even if remote, she certainly wouldn't have hugged that small child. She would still be in bed, weak and unable to campaign.
I'm not convinced that pneumonia was the cause of her collapse last Sunday morning.