For centuries people with maladies of any kind could look forward to a good dose of mercury, as the medical establishment had pretty much concluded that shiny things were good for people. This shipwreck made them think again.
Source: This Shipwreck Gave Doctors the First Clue Mercury Might Not Be Good for You
Brief exposure to a limited amount of ethyl mercury (the silver stuff most people know) is usually fine. You have to stay around a lot of it in a non-ventilated space, like miners or hatmakers (‘mad hatters’) used to do, before the vapor becomes a problem.
Methyl mercury, on the other hand, is basically armageddon. We learned about that thanks to the many human sacrifices in Minimata, Japan; the story on that event would be graphic enough that it would probably need a lot of warnings for the reader.
Methyl mercury shows up in coal emissions and certain industrial processes. It usually exits a smoke stack in very small amounts and spreads evenly around the environment. Normally that would be fine… Except that when methyl mercury meets something biological, it sticks. By weight most things are plants, so right off the bat it mostly sticks to plants. When things eat those plants they mostly digest and excrete the plant bits, but the mercury stays. So herbivores keep all the methyl merury from all the plants they ate. That still adds up to not that much mercury. But then something eats the herbivores and it keeps the mercury from all the herbivores it eats. Two steps of concentration (‘bioaccumulation’) starts adding up to real numbers.
That still does not pose much of a problem on land. Not many things eat carnivores that eat land herbivores, so the mercury only concentrates by one or two steps. But the sea has a lot of trophic levels, so you can have four or five steps of bioaccumulation. That is why kids should stay the hell away from top predators like tuna.
My Warfarin Diet 