Mercury is prevalent in/near mining areas where gold was or is mined, because they used it to amalgamate the gold to collect it all and then the mercury off. Some processing plants still use mercury and its still used in the third world for mining and processing.
(If you have a gold ring and touch mercury, the mercury will stick to the gold and not come off, or "amalgamate". You have to boil it off which is BAD. I know a professor who accidentally got mercury on his gold wedding ring and basically had to send it to be disposed and get a new ring).
Its gets into the water and accumulates in fish and other organisms. Fish travel, and the mercury gets around.
In brazil, they still mine gold with mercury, so I'd advise against eating fish from the rivers of brazil.
Mercury reagents are often much more dangerous that the metallic mercury.
If ever you are in a lab that uses alkylmercury reagents (ie methylmercury derivatives), be very careful. That stuff kills and there is practically no way to stop it. It can be rapidly absorbed through the skin or inhalation.
Here's a case of a researcher at Dartmouth who died from mercury poisoning from alkyl mercury reagents:
http://collaborations.denison.edu/naosmm/topics/dartmouth.html
The mercury in thermometers/barometers is safer, but can be toxic over time, and humans/animals/bacteria actually make it more toxic in by methylating the mercury in vivo.
Here's a site with some info on testing and treatment of heavy metal poisoning:
http://www.beatcfsandfms.org/html/CheckForHg.html
Heavy metal poisoning can often be misdiagnosed at low levels, its not a bad idea to test once in a while if you are feeling crappy for long periods of time.
Also, your local water company usually montors the local water and reports on the metal content. Check with them periodically.