Short answers, pulled from the story.
Unit 731 was a secret research facility operated by the Imperial Japanese Army between 1933 and 1945, located in the Pingfang district of Harbin in the Japanese puppet state of Manchukuo, now part of Northeast China. It was led by Lieutenant General Shiro Ishii and was officially known as the Epidemic Prevention and Water Purification Department of the Kwantung Army. The complex covered 6 square kilometers and comprised more than 150 buildings.
An estimated 14,000 people were killed inside the Unit 731 facility itself. Biological weapons developed by the unit caused the deaths of between 200,000 and 500,000 additional people in Chinese cities and villages through deliberate contamination of water supplies, food, and agricultural land. A 2002 international symposium in Changde estimated the total number killed by Imperial Japanese Army germ warfare and human experiments at around 580,000.
Ishii was granted immunity from prosecution by American occupation authorities in exchange for providing exclusive U.S. access to his unit's biological warfare research and human experimentation data. General Douglas MacArthur secretly authorized the arrangement after Lieutenant Colonel Murray Sanders threatened Japanese officials with Soviet involvement in the investigation. The Truman administration concealed the unit's crimes and paid stipends to former personnel.
Prisoners, referred to internally as "logs," were subjected to vivisection without anesthesia, deliberate infection with plague, cholera, typhoid, syphilis, and other diseases, frostbite experiments, exposure to chemical weapons including mustard gas, incompatible blood transfusions, limb amputation, and organ removal. No documented survivors are known. All remaining prisoners were killed when the unit was disbanded in August 1945.
The Khabarovsk war crimes trials were held in December 1949, when the Soviet Union prosecuted 12 former members of Unit 731 and its affiliated units. Sentences ranged from 2 to 25 years in Siberian labor camps. The lead prosecutor was Lev Smirnov, who had previously served as a top Soviet prosecutor at the Nuremberg Trials. All defendants had returned to Japan by 1956, and the United States dismissed the trials as communist propaganda.
In August 2002, the Tokyo District Court ruled for the first time that Japan had engaged in biological warfare. Presiding judge Koji Iwata ruled that Unit 731, under orders from Imperial Japanese Army headquarters, used bacteriological weapons on Chinese civilians between 1940 and 1942, spreading plague and typhoid in the cities of Quzhou, Ningbo, and Changde. Victims' compensation claims were rejected on the grounds that they had been settled by international peace treaties.