Minamata disease, aka methylmercury poisoning, was recognized in 1953 among the inhabitants of Minamata Bay in southwestern Kyushu, Japan. It occurred mostly among fishermen and their families.
A factory located on the bay started producing vinyl chloride and actaldehyde in 1952. Both production processes involve the use of large quantities of mercury, which were discarded into the bay. The illness first appeared in 1953, but it took until 1956 to diagnose is as metal poisoning from fishing products. By then 2000 people were affected, 43 died and 700 of the survivers were left with severe permanent disabilities. Fishing was banned by 1957, which stopped the epidemic. It took however until 1959 to show that mercury was the toxic element involved, and until 1960 to prove that the source was the factory effluent which was discharged directly into the bay. 
In addition to the methylmercury produced by anoxic bacteria, 5% of the mercury effluent form the factory was already in the form of methylmercury. During the investigations of 1959 it was shown that the sediments contained 200 ppm mercury, bivalves 10-39 ppm (dry weight), and fish in the bay contained 10-55 ppm (dry weight) of mercury, most of it in the form of methylmercury.
- Clark, R,B., 1999. Marine pollution. Oxford University press, Fourth edition, pp 161