the dirty secret is that it doesn't in a lot of cases, that is why foodborne illnesses are sky high from canned fish. that is why i catch my own fresh fish.. fish that is 3 days old or more is no longer considered fresh.
you won't find me ever eating fish out of a can or buying frozen fish.
if you do eat canned or frozen fish i highly recommend smelling it and tasting a small amount before eating.. if it tastes or smells funny throw it out
[h=2]Bacterial Contamination[/h]As with other foods, there is a risk that canned tuna can be contaminated with pathological bacteria, such as Clostridium botulinum, Salmonella, Shigella, Campylobacter jejuni and Escherichia coli, which can cause food poisoning and even death. Symptoms can include diarrhea, vomiting and stomach cramps four to 36 hours after ingestion. In June 2010, Tri-Union Seafoods, the parent of Chicken of the Sea, recalled 1,105 cases of white tuna packed in water tuna. The FDA said the tuna was being recalled because of possible pathological contamination due to loose seals and seams in the cans.
[h=2]Histamine[/h]Histamine poisoning, also called scrombroid poisoning, can occur if the canned tuna isn't handled properly after it's opened. Histamine forms when the free amino acid histidine and bacteria inherent in the tuna interact. Once the can is opened, the tuna should be eaten within two hours or refrigerated. The Center for Food Safety for Hong Kong's Special Administrative Region issued an advisory on histamine poisoning, and in the United States, the Centers for Disease Control and Prevention has looked at cases of histamine poisoning in fresh tuna. The FDA reports reported incidents of histamine poisoning have declined since the initial reports.