A bioterrorist cell would not have the resources to craft a perfectly engineered biological weapon. They'd be more likely to have access to limited quantities of some pre-existing biological agent (like anthrax, say). So what you'd see in that scenario is attacks that cause a bunch of casualties in a few locations in one big splash, and then quickly peter out after that.

It's like, what's the difference between a terrorist nuclear attack and a full-scale nuclear war? In the former scenario, you're talking a small number of low-yield devices triggered in one country; the rest of the world still stands able to provide assistance. In the latter scenario, there's nobody left to help who hasn't also been blown to ashes.