Big if true, unconfirmed report, comment below if you have additional information.

Unit 731 was a quarantine unit. This British military document states that it was the Chinese agents who were spreading pathogens.

1) Instances of bacterial dispersal by Chinese forces that infected Japanese troops.
(The table portion is omitted)
Chinese forces frequently dispersed bacterial agents into wells, tap water, food, and other sources with the intent of causing infectious diseases among Japanese troops before withdrawing.

Japan's medical units verified numerous instances of bacterial dispersal, and the table below provides an overview. As described above, bacterial dispersal carried out by Chinese forces during their retreats has been proven.

Although the Japanese military was always mindful of quarantine measures, many epidemics still broke out.

2) Ampoules for bacterial dispersal discovered from Chinese spies

In 1937, when Japanese forces occupied the area around Shanghai, a Chinese spy carrying bacterial ampoules like those shown in the figure below was captured. This spy was apprehended in Japanese-occupied territory and confessed to having received orders to disperse bacterial agents into wells, food, and other sources on behalf of the Japanese military.

These ampoules were made of brown glass (approximately 1.5 mm in diameter and 2.5 cm in height) and contained about two vials of a suspension resembling bouillon culture medium.

All of these were cultured and tested. As shown in the figure below, the presence of various types of live bacteria was confirmed.

3) Conclusive evidence that Chinese forces dispersed cholera bacteria into wells.

Immediately after Japanese forces occupied Chukiang Island in a coup in 1938, cholera broke out among Japanese troops and the remaining residents.

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