In 1908, the Chisso Co. Ltd opened. It was a factory that manufactured nitrogenous fertilisers which soon became a massive industry in the small town of Minamata, in Kyushu.
mercury + mercury methyl
Unbeknownst to everyone, the factory waste that was released into Minamata Bay from 1932 contained mercury, which is toxic when ingested in high concentrations.
Certain bacteria in the water converted some of the mercury into methyl mercury, which is a compound and even more dangerous than the element on its own.
Some fish consume the mercury waste. When prey are eaten by their natural predators, the mercury travels up the food chain and concentrations become dangerously high.
Fish is one of the staple foods in Japan and the majority of the population consumes it multiple times a day. In the 1950s, many of the fish in Minamata Bay died of mercury ingestion whilst others were caught and locally consumed.
Cats and birds began behaving strangely, with lots of jerking movements and odd actions. They eventually began to die but no one was able to understand why this was happening because they didn't make a link to the fact that both animals were heavy fish consumers.