The submarine had space for three specialized Japanese airplanes, called Seiran, which translates literally to "storm out of a clear sky." Before the Japanese surrender, this particular submarine's original mission had been to secretly sail westward from Japan to the US east coast, where an attack would be unexpected, and use its three aircraft to drop rats and fleas infected with bubonic plague, cholera, typhus and other diseases upon New York, Washington D.C., and other cities along the eastern seaboard. When problems made that plan infeasible, the sub was retasked to bomb the Panama canal from the east, but the end of the war arrived before the crew could carry out its mission.
By the end of World War 2, Japan had done quite a bit of experimentation with germ warfare, mostly in the form of infected fleas. The program got its start in the 1930s when Japan occupied Manchuria, and later in their invasion of China. These biological weapons were developed at Japan's Unit 731, an installation disguised as a water purification plant. The Allied forces had long suspected that Japan was utilizing germ warfare against China, but was unable to conclusively prove their suspicions during the war.
When America was attacked by Japanese balloon bombs, US officials were concerned that these might include some of Japan's infected flea payloads, but no such biological balloon bombs were ever discovered.
Several epidemics of cholera, typhoid, anthrax and bubonic plague were reportedly caused in China by Japan's "Uji" bombs, which were designed specifically to burst hundreds of feet above the ground, and rain infected fleas upon the populace. By some estimations, these attacks triggered outbreaks which killed as many as 50,000 Chinese people over six years. According to Chinese reports, infected houses, hospitals and other buildings were burned and had to be left untouched for decades, and fears of further outbreak still haunt the cities today.