did you know that north korea offered to have all 535 congressmen visit their anthrax factory?
To be honest I'm more afraid of a bio weapon than a nuke. At least nukes in the form of a missile can be intercepted. smaller nukes like say a small one in a suit case is pretty low yield and more like a dirty bomb. Even planes carrying bombs can be intercepted.
However if a bioweapon goes off it contaminates everything from water/fluids, food, air, can live/thrive/and spread as long as they can find a host or surface to survive on. Plus it's cheaper to engineer microbes in a lab than build a nuke.
Agro-terrorism is also a big concern too that a lot of people overlook.