Subsequently, the Japanese command contemplated a biological attack utilising plague bombs from Tokyo and Kwantung Army.

The Ministry of Agriculture and Forestry thus began organising rat breeding in the Japanese prefectures of Ibaraki, Tochigi, and Chiba.

[1]: 42–43 In early 1942, the Japanese military established the pioneer team of the Epidemic Prevention and Water Purification Department of the Southern Expeditionary Army Group in Nanjing, China.

[3] During the Japanese occupation, Dr. J. M. J. Supramaniam also collaborate with Unit 9420 but privately sold medical supplies to needy prisoners of war and hospitals.

[7][8] From April to June 1943, the unit was temporarily stationed in Thailand to provide services for the construction of the Thai-Burma railway before returning to Singapore.

[1]: 27–28 In November 1944, the senior leadership of the Japanese Imperial General Headquarters realised their inability to halt the American offensive and decided to employ bacteriological warfare to demoralise the enemy.

Additionally, the Japanese military used animal specimens from the Raffles Museum for research, considering hamsters, squirrels, and guinea pigs as supplements to rats.

[3] In addition to the plague, laboratories in Outram, Singapore also researched cholera, malaria, smallpox, typhoid fever, dysentery, and anthrax.

[3] In Malaysia, Unit 9420 occupied Tampoi Mental Hospital in the northwest suburbs of Johor Bahru near Singapore to breed rats and develop biological weapons.

[10][14] On the other side of the laboratory was an isolation wall with a 24-hour operating boiler outside to burn the bodies of rats and provide boiling water for disinfection.

[17] In northern Burma and western Yunnan, China, the Southern Army Unit 9420 Epidemic Prevention and Water Supply Department was stationed to collect and breed rats and cultivate bacteria.

[2] At Rabaul, Japanese military doctor Einosuke Hirano was witnessed to have experimented on prisoners of war from the United States, Australia and New Zealand.

[1]: 95–96 In the meantime, Japanese forces also recruited Taiwanese, disguised as students, to release cholera bacteria into ditches, wells, and ponds along Burma Road.

[1]: 119–123 Subsequently, they bred and released infected rats through epidemic prevention and water supply units in places like Mangshi and Tengchong, causing an outbreak of plague.

[1]: 188 After the failure of the Guadalcanal Campaign in 1943, the Japanese military began planning to launch bacteriological warfare on the Pacific front but none were executed.

[1]: 45, 48 On 26 April 1944, at a meeting of the Army Ministry Directors, Chief of Staff of the Operations Department, Hattori Takushiro, proposed using submarines to launch plague attacks on Sydney, Melbourne, Hawaii, and Midway Island in an attempt to regain strategic advantage in the Central Pacific.

[2] When younger doctors learned about their history, they were shocked, and some formed non-governmental organizations to publicly disclose their understanding of what happened back then.

[21][9] In November 2017, Singaporean researcher and collector Lim Shao Bin gave a lecture on the topic at the National Library of Singapore, once again drawing attention.