Most bio and chemical weapons have a strictly limited period of effectiveness once released. I am highly skeptical of massively diluted bio-agents being capable of causing disease after weeks/months of being exposed to oxygen, ozone, heat, UV, etc. in the air.
There were articles posted about anthrax that stated that it could be expected to survive for "months" on the isolated islands where it was tested. It seems to be quite hardy.
On the other hand, I do wonder about the dispersion.