Anthrax
On April 2, 1979, there was a leak at one of the plants just outside of Sverdlovsk that made anthrax during the Cold War. 94 people were infected and 64 of those people died. The USSR blamed the deaths on another strain of anthrax that the people caught by digesting infected meat. However, later it was revealed that the outbreak was caused by an accidental release of the bacteria from a nearby plant which developed biological weapons for the USSR.