So mercury is a heavy metal. It’s that stuff in old thermometers, and it’s what the Terminator is made out of (I think). You get mercury poisoning from exposure to too much mercury (no kidding). It causes problems for the kidneys, brain, and lungs. According to wikipedia, the symptoms include an itching and burning sensation on the skin, skin discoloration, swelling, and skin shedding (wait, what?) So, also according to the wikipedia article, there is mercury residue from processing in high fructose corn syrup. I don’t know how accurate that is, and haven’t heard of it before, but it’s probably worth looking into. I guess you can also breathe it in from coal plants and volcanoes.
It affects the nervous system, and can also cause problems with gums and teeth. Long-term exposure can result in brain damage or death. It’s almost always toxic for fetuses and infants. It can cause serious birth defects. It sounds like it totally sucks all around. And you can’t just figure out that you have it. It is detected in a blood or urine test.
So just don’t over do it with the tuna.