Plague was used as a bacteriological weapon by the Imperial Japanese Army. These weapons were provided by Shirō Ishii’s units and used in experiments on humans before being used on the field. In 1940, the Imperial Japanese Army Air Service bombed Ningbo with plague-carrying fleas. During the Khabarovsk War Crime Trials, the accused Major General Kawashima testified that, in 1941, 40+ members of Unit 731 air-dropped plague-carrying fleas on Changde, causing epidemic plague outbreaks.
Bubonic plague infection causes tiny blood vessels in the hands and fingers to clog up and cut off circulation. Without blood, the flesh dies and turns black (called "gangrene"). This is why in the Middle Ages bubonic plague was called "the Black Death." In the 14th century it killed an estimated 25 million people, or 30–60% of the European population.
During the Bubonic Plague, doctors wore these bird-like masks to avoid becoming sick. They would fill the beaks with spices and rose petals, so they wouldn’t have to smell the rotting bodies. A theory during the Bubonic Plague was that the plague was caused by evil spirits. To scare the spirits away, the masks were intentionally designed to be creepy.