They may not have ever officially recognized it, but the Soviets had an accidental release of anthrax in Sverdlovsk in 1979. About 100 dead.
Reading the history of those things, it appears the Soviet biowarfare program made it's major advances whenever they had an accidental leak.
But it ONLY managed to effect 1/3rd because we don't care!
Wat
It's had how many hundreds of years, with us not giving a shit, and it's still only 33% effective.
Then again, it's a rather impressive number for a parasite that doesn't target humans in the first place. Getting from one continent to the other is quite a feat for a microbiological beasty.
How effective can you get a bioweapon?
Extinction of most mammal life.
Anthrax airbombing will kill all life in a certain area, and make it inhabitable for centuries. (40 years with cleanup). And that's a virus that is chosen to stay local. More dangerous viri exist and can easily be developped. Scientist managed to make an aerial variant of the pig/ bird flue in a few months, so a dedicated team given a few years will come up with something absolutely destructive.
There's also the fungi that spreads on trees, and then grows in peoples lungs. If you capture it, select for infectability and then drop in all forests, it will slowly kill all human life.