Far as I know, having seizures is not part of having pneumonia.
Maybe not, but there are underlying conditions that can cause seizures AND can also weaken the immune system, which increases susceptibility to pneumonia.
What if Hillary knowingly entered the presidential race with a debilitating chronic condition (Parkinson’s, ALS, etc.), and subsequently contracted pneumonia?
She and her doctor would much more willingly admit to the pneumonia, than to the underlying disease.