<p>I believe it is mercury poisoning. Korra even suffers the symptoms:
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Minamata disease (Japanese: 水俣病 Hepburn: <i>Minamata-byō</i><sup class="t_nihongo_help noprint" style="line-height:1;"><span class="t_nihongo_icon" style="color:rgb(0,0,238);font-weight:bold;font-size:9px;line-height:normal;padding-right:0.1em;padding-left:0.1em;">?</span></sup>)<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, sometimes referred to as Chisso-Minamata disease </span><span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">(チッソ水俣病 <i>Chisso-Minamata-byō</i><sup class="t_nihongo_help noprint" style="line-height:1;">
<span class="t_nihongo_icon" style="color:rgb(0,0,238);font-weight:bold;font-size:9px;line-height:normal;padding-right:0.1em;padding-left:0.1em;">?</span></sup>)</span><span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, is a </span>
neurological<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;"> syndrome caused by severe </span>
mercury poisoning<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">. Symptoms include </span>
ataxia<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, </span>
numbness<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;"> in the hands and feet, general </span>
muscle weakness<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, narrowing of the </span>
field of vision<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, hallucinations, and damage to </span>
hearing<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;"> and </span>
speech<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">. In extreme cases, </span>
insanity<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, </span>
paralysis<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, </span>
coma<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;">, and </span>
death<span style="color:rgb(37,37,37);font-family:sans-serif;font-size:14px;line-height:22.399999618530273px;"> follow within weeks of the onset of symptoms. </span>
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