Even in normal times, there was always tongue-in-cheek the concept of the "Kongressseuche" (congress plague), because many people got (mildly) ill with a cold or the flu. :)
Other years there were colds... The usual... But the 2019 edition was real nasty.
I think the "Seuche" in Congress-Seuche falls somewhere in between. I can attest that at least Congress-Seuche has been in use in the past, somewhat analogous to the annual "Wiesnseuche" flu wave in Munich during and after the Oktoberfest.
It might have been this article, the earliest (relevant) mention of pneumonia cases I could find: https://news.ycombinator.com/item?id=21926713