One of the best bioweapons is Tularemia, and part of that reason is its general lack of lethality. It temporarily renders the exposed population incapable of putting up a good fight, but it kills relatively few. Escape isn't that big a deal (because again, it's not very lethal), and it's easy enough to inoculate one's own troops against the particular flavor being used.
Not that I think it was engineered as a bioweapon, but it does show that bioweapons don't have to be deadly to be effective.