Unit Oka 9420 , also known as the Epidemic Prevention and Water Purification Department of the Southern Expeditionary Army Group , was a disease research unit within the Japanese army. Founded in Nanjing, China in 1942 and headquartered in Japanese-occupied Singapore, the unit had branches in Malaya, Indonesia, the Philippines, Papua New Guinea, Thailand, and Burma, and participated in bacteriological weapons attacks in northern Burma, Yunnan, China, and Papua New Guinea. With a tropical climate, suitable for the breeding of rat fleas, Malaya was the largest of the rat flea farms outside of Japan and China during World War II. The unit left Singapore in mid-1945 and dissolved in 1946.