The video of her freezing up and then falling as she tries to enter the van looks, to me, exactly like my FIL who just passed away from Parkinson's.
However, if Parkinson's is the problem, I don't see why you would want to keep it a secret. It is a very slowly debilitating disease and it doesn't affect one's judgment, just muscle control.
I don't know exactly why Clinton collapsed, or if she has Parkinson's disease or not. But I'm pretty sure that electing a president with dementia would lead to some very big problems in this very dangerous world.