"Likely" doesn't mean it's actually going to happen. Even if it does, it's far less probable than a bunch of guys doing a suicide bombing mission or a raid on a scale of what just happened in Mumbai a few days ago.

Biological weapons, at least the effective ones, are not so easy to use for a terrorist group. They also have the tendency to work both ways so there is a major risk of self-infection. I guess it's a lot easier and more bang for the back to just build a big bomb or spray random people full of lead with an AK-47.