Fish and molluscs (shellfish) comprise a portion of the diet of many. Fish on the whole are safe, but there are certain species to err on the side of caution before consuming. Origin is another concern.

Ciguatera

Ciguatera is an "umbrella" term for a ichthysarcotoxism, a medical term for "fish poisoning". This food poisoning is caused by the consumption of tropical and subtropical marine fish which have accumulated naturally occurring toxins through their diet. These toxins originate from several species of the dinoflagellates (algae, mainly the species of algae, Gambierdiscus toxicus) which flourish in these regions. Perhaps a more correct name for this malady is "ciguatoxinosis" or toxism, since ciguatera denotes the poisonous snail (Species originally, Taino), and not the toxin (ciguatoxin). Regardless of my opinion, the fact is this toxin accumulates in the tissues of several fish (barracudas, groupers, snappers, jacks, mackerel, triggerfish and probably other species). If these fish are eaten and happen to contain enough toxin, the consumer can become severely ill. I had the personal pleasure of living in the Caribbean (West Indies) for several years, and witnessed several people becoming very ill after mealing on a tasty barracuda. This toxin (the ciguatoxin) affects cellular sodium channels. Basically, sodium channels regulate sodium concentrations. If sodium is not properly regulated, any function dependant on sodium, logically goes awry. This is includes, most areas of the body, and individuals ingesting this toxin can develop arrythmias (irregular heart activity), neurological abnormalities (abnormal sensations), and other symptoms. Although there haven't been many deaths, there have been some. I'm not suggesting to boycott tropical fish, but rather adding guidance in choosing certain types in regard to location and the risk involved.