Though Legend of Korra is a show about elemental bending set in a fictional world, the series takes a lot of influence from reality. With the Red Lotus and their poison, the show never confirms that it’s mercury, but all signs point to it. Liquid mercury is highly toxic, and would require a skilled metalbender to manipulate. The poison shares the same appearance as mercury, and causes a physical and mental reaction in Korra that includes hallucinations.
In season 4, episode 2, “Korra Alone,” the show recounts the next few years, as Korra recovers. Katara (Eva Marie Saint) attempts to heal her, but she tells Korra that the poison did a lot of “internal damage.” Korra’s symptoms strongly match up with that of Minamata disease, as described by Boston University, a neurological illness that occurs after severe mercury poisoning. People usually experience ataxia — “a degenerative disease of the nervous system,” as explained by the National Ataxia Foundation — a lack of coordination and balance, as well as muscle weakness and numbness in their limbs, all of which happens to Korra.
Korra spends a lot of time working hard to regain control of her limbs, and move on from the wheelchair. The poison also affects her bending, and causes depression and PTSD that she must work through. All of these symptoms line up with Minamata disease.
Altogether, the show takes the subject of mercury poisoning very seriously, and it seems safe to say that Minamata disease likely played a huge influence in the depiction of Korra’s intense experience.