I don't think that the Aflatoxin + Bonewall + Epidemic combo would be too OP, because it would take a long time to set up properly.
You'd need at least three cards to make it work, and even fully upgraded it wouldn't be a cheap combo to play. You'd also have to play them in the right order.
And once you played it, there would still be a delay of a few turns before you really saw any benefit.
Example:
I have three malignant cells on my side and a bone wall with 5 charges. My opponent has a fire dragon in play. I play outbreak on his/her fire dragon.
It will be three turns before the dragon dies and outbreak spreads.
It would make for a useful combo, and a nice thing to add to many death decks, but by no means is that overpowered.