The only nice thing about biological warfare is that the most effective biological weapon that some foreign rogue state could conceivably produce would not discriminate between belligerents. It would have been poetic justice if the Soviet Union had a biowarfare "Chernobyl".
They did. Sverdlosk.
Actually, the Soviets DID have a biowar version of Chernobyl, and it featured our old friend anthrax. They accidentally released a mess of the stuff into the air in the city of Sverdlovsk. Hundreds died. They told the West that it was an outbreak caused by infected meat. Liberals believed them; conservatives said it was bunk. Of course, the conservatives were right. The Russians have since admitted they were making anthrax at Sverdlovsk.
The story is recounted in Ken Alibek's Biohazard. He even talks of having a shame-faced man come in for a job interview at his germ warfare facility. Turns out he was the guy who'd caused the Sverdlovsk disaster. It was his job to replace the filters that removed bacteria from the air before pumping it out of the building. Well, he removed the old filters all right, but forgot to install the fresh ones. And so for a period of hours, anthrax-tainted air flooded the city. Utterly appalling story. By all means read the book.