Anthrax, by itself, is nasty, but it’s nasty in ways that make it unsuited for a ballistic missile warhead. Ballistic trajectories require the missile to travel through space and back to Earth, and if there’s any failing in the heat shield, then the anthrax itself will be rendered inert, as anthrax is sensitive to heat. It needs to find a pathway into open lesions, so when used as a deliberate attack on humans, it works better if placed where a person will inhale, rather than just blasted through the air. For these reasons, anthrax is a terror weapon that works better in envelopes than a weapon of war distributed by explosion. And, finally, it doesn’t really matter what the payload of an ICBM is; any country attacked by an ICBM will treat it as a nuclear weapon and respond accordingly.
I strongly disagree with the last part about treating any ICBM as a nuke.
Sure, in regard to anti-missile defenses if you see a missile heading toward your capital you will try to shoot it down on the assumption it could be nuclear.
But unless you see lots of missiles heading toward you and you have a launch-on-warning nuclear posture (because you fear lots of missiles could knock out your nuclear force), you won't fire nukes in retaliation until you know it is a nuclear warhead.
You'd ride it out because letting the missile hit (if you miss with anti-missiles or don't have anti-missiles) won't destroy your retaliatory capacity.
That's one reason we have a nuclear triad of land-based, submarine-based, and aircraft-carried nukes. It helps to make sure enough nukes survive a massive nuclear surprise attack so we don't have to launch-on-warning.
