Mercury poisoning is the common phrase for mercuralism, a condition caused by the body absorbing mercury, either in its elemental form or in a compound. Mercury is a heavy metal, liquid at room temperature, and is easily absorbed through the skin. It is a neuro-toxin that can severely damage the human nervous system and brain.
Mercury poisoning was commonly seen in hat-makers in the 18th century, since a mercury compound was widely used in making felt. Unaware of the danger of skin absorption of mercury, hatters handled the felt, and over a period of time, went insane from mercury poisoning. This gave rise to the phrase "mad as a hatter," which in turn led to Alice in Wonderland's Mad Hatter character. Mercury poisoning is less prevalent than it was in those bad old days, but can still be a concern. The most common source of high levels of mercury today is consumption of mercury-contaminated fish.
Mercury in the ecosystem is almost impossible to remove, and once a fish habitat has become contaminated, the fish that live there absorb it. Most fish don't live long enough to develop a significant load of mercury, and hence aren't dangerous to consume. Long-lived predator fish such as sharks, however, not only absorb mercury from their environment; they also acquire the mercury burden of all the fish they consume. Therefore, shark and other predator fish might pose a risk of mercury poisoning if consumed.
That said, the level in even an older shark isn't significant enough to harm an adult. The risk is greater for children, or for pregnant women who might pass the mercury poisoning on to their unborn child. It is recommended that pregnant women or women planning to become pregnant refrain from eating certain types of fish known to have higher than normal concentrations of mercury.
The human body can not, unaided, process and remove mercury from the brain and nervous system. Therefore, in cases of mercury poisoning, radical therapies are required to eliminate the contaminant. Chelation therapy is the therapy currently used, in which chelating agents are introduced. Chelating agents can form bonds with the poisonous heavy metals, and then the compound created can be eliminated.
A recent controversy involving mercury poisoning is the claim by parents of children with autism that their child's condition was caused by childhood immunization shots containing a preservative that included a mercury compound. The Centers for Disease Control have denied that a link exists, but circumstantially, autistic symptoms often appear shortly after a child's one- or two-year immunizations, and the condition is much rarer in populations that do not give childhood shots. Drug companies are no longer preserving their drugs using the mercury-containing compound, but it couldn't hurt to ask before your child's regularly scheduled shots.