Using biological warfare was banned by the Geneva Protocol in 1925, but Japan rejected the ban. If germ warfare was effective enough to be banned, it must work, military leaders believed. Unit 731, a secret unit in a secret facility — publicly known as the Epidemic Prevention and Water Supply Unit — was established in Japanese-controlled Manchuria, where by the mid-1930s Japan began experimenting with pathogenic and chemical warfare and testing on human subjects. There, military physicians and officers intentionally exposed victims to infectious diseases including anthrax, bubonic plague, cholera, syphilis, typhus and other pathogens, in an effort to understand how they affected the body and how they could be used in bombs and attacks in WWII.
In addition to working with pathogens, Unit 731 conducted experiments on people, including — but certainly not limited to — dissections and vivisections on living humans, all without anesthesia (the experimenters believed using it would skew the results of the research).
Many of the subjects were Chinese civilians and prisoners of war, but also included Russian and American victims among others — basically, anyone who wasn't Japanese was a potential subject. Today it's estimated that about 100,000 people were victims within the facility, but when you include the germ warfare field experiments (such as reports of Japanese planes dropping plague-infected fleas over Chinese villages and poisoning wells with cholera) the death toll climbs to estimates closer to 250,000, maybe more.
Believe it or not, after WWII the U.S. granted immunity to those involved in these war crimes committed at Unit 731 as part of an information exchange agreement — and until the 1980s, the Japanese government refused to admit any of this even happened.