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From http://www.epa.gov/mercury/effects.htm.\\
Other Risks:
The fish may have bacteria, virus, parasites and other disease causing organisms. In most cases there are no regulations to verify the fish catch for such stuff. Sometimes the fish go directly from the fishermen to the consumers without any sort of regulating body. sometimes the fishermen themselves are the consumers which complicates the situations further.
Of late there was an global breakout of SARS which spread with the chicken. If such a problem comes up with fish it would take a longer time to figure out because there is a lot less number of body regulating fish than chickens. Every commercial chicken farm needs to be registered with some regulatory body while it is not at all necessary for the fishermen to register themselves with any regulatory bodies in some cases.
The reason behind the fish carrying diseases does not necessary from the fish themselves but it might come from the ocean and the water in which the fish live which does not have any boundaries.
Mayo Clinic. (2007, Nov 21). Fish FAQ: The merits and hazards of eating fish
Retrieved Nov 21, 2007, from the World Wide Web:
http://www.mayoclinic.com/health/food-and-nutrition/NU00292