A scourge means something that causes much pain and suffering, like the terrible diseases which Henrietta's unwitting, or unknowing, contribution helped eradicate.
Then he used a word I'd never heard before: Illuminati. It sounded exotic, powerful, whatever it was. Grandma, he said, was an unknowing agent of the Illuminati.
For those conversations, so brilliant, so scholarly, and so consciously unknowing, seemed to me, silent in the background, to reveal the truth that even college graduates did not know anything, really.