Scalability is a bit of an issue, but a biological weapon like weaponized anthrax or chemical weapon like a powder that converts to 4highly effective nerve gas could conceivably delivered by methods similar to the nuclear triad. Strategic bombers could airdrop them over population centers, and with enough engineering we could probably make ballistic missiles with payloads that disperse such agents in an air burst, using a small amount of explosives to scatter it over an area the size of Manhattan
WW2 showed that strategic deterrence with chemical and biological weapons doesn't work. Both sides feared gas particularly and therefore didn't use war gas on each other (civilians is another matter.) Germany had very potent nerve gasses and had reason to believe the allies did as well, and didn't dare use them. But the threat of these gasses wasn't strong enough to deter the rest of the war.
