-_-
This would add nothing to the game... the number of nuclear weapons in the game is already questionable, but each does fill a unique gameplay role:
Strategic Bombs create a disaster in a 2 country radius.
Nuclear Missiles create a disaster in a 1 country radius.
Tactical Weapons create a disaster in only the attacked country.
Submarine Missiles add unlimited range to nuclear capability.
Chemical Missiles allow weaker disasters without requiring the weapons grade uranium + plutonium supply line.
A biological disaster would function exactly the same as a Chemical disaster, Nuclear disaster, or Earthquake. These are all essentially the same thing, slapping on a new name and size adds nothing to gameplay. There is no role for it to play... a non-plutonium supplied Nuke-power-level weapon is completely imbalanced anyway... and that sounds like what you're proposing.