The closest we’ve really come is the CDC and Eugene’s bullshit story, so this would be as big a narrative game-changer as Alexandria and Negan were for The Walking Dead proper, and the implications could branch out infinitely.
I mean, technically Nick could just kill Alejandro at the beginning of next week’s episode, thus ending this thread without an explanation, but I don’t imagine that would happen.
Because in the 13 years that Walking Dead creator Robert Kirkman has been telling this universe’s core story in the comic books, and in the going-on-seven years the TV show has been on the air, never has there been any sign of cures or immunity where the zombie epidemic is concerned.