The Bubonic plague, also known as the black death in Eourpe which killed tens of millions of people in the 1340s.
In China during the World War 2, the chinese government was testing this plague on chinese civilians and solders, which killed thousands. The Chinese government was spreading the plague through flees, which they let eat at the test subject (chinese civilans) then they would let them run and bit other chinese who wern't being tested on.
Someone who is infected with the bubonic plague could die with in 3 to 7 days without treatment. When getting the treatment you are injected with several types of antibiotics to kill the plague off.