Exposure of high-level support of the Black Hand combined with a surviving Tsarist Russia carrying out some horrible acts against newly conquered peoples?
There's one thing to mention the flu and another to mention the WAllies using the flu as a weapon of mass destruction.
That doesn't explain how he gets to power. He was only in a position of authority in the Nazi regime due to the patronage of Himmler and to a degree Canaris. With no Nazis there is every chance Heydrich remains a talented violin player who got kicked out of the Navy for having an affair.
Possibly, I did not think giving an example might predicate explaining how the rise of a demogogue might cone to pass. My original post is exited to reflect this.
I still do not see any earlier reference to an earlier Spanish flu though.