As the title would indicate, your challenge is to create a world where biological weapons are regarded as being just as bad, or even worse, than nuclear ones. The most obvious ways I see to do this would be to have worse, or more, bioweapons used in warfare and/or not having nukes actually used against somebody. If you can think of a different way to do it, well, you can have some of those bonus points that seem to be laying around.
On a related note, what kind of bioweapon would need to be created [and used] in order to have it been seen like this? I mean, some of the OTL ones are pretty nasty but I'm not sure that if any of them could be considered 'worse than nukes' but that might just be my relative ignorance of the subject talking.