Biological terrorism is the 800 pound (440kg) gorilla in the room.
It is very hard to defend a population from bioterrorism.
The reaction in retaliation would be decimation of the offenders political entity or sovereign state -- if a world power was the bioterrorism target. Any serious bioterrorism event would make the conflicts in the Middle East look like a trip to Disneyland
Hopefully, this is sufficient deterrence.