There are a variety of parasites that salmon can be infected with, the most common being the heterotrichous nematode, which can produce acute food poisoning if eaten in relatively large quantities at one time; even if eaten sparingly, it can lead to the body being allergic to nematodes in the future. As parasites that thrive in cold water, heterotrichous nematodes are not difficult to kill. Under 60 degrees, for example, they will be killed in seconds. But for those who prefer to eat them raw, heating them up to that temperature is unacceptable, and the only way to freeze them is through cold temperatures.

As animals somewhat higher than bacteria, they can indeed be frozen to death, just under slightly harsher conditions. EU regulations state that fish must be stored below -20 degrees Celsius for more than 24 hours before they can be used for raw food. They believe that such treatment will adequately kill these parasites. The US has no mandatory rules, only a recommended 'operating guideline'. And that recommendation is much stricter than the EU requirement, which is -20 degrees Celsius for more than 7 days, or -35 degrees Celsius for more than 15 hours.

It is clear that the US guideline kills nematodes more adequately than the EU requirement, but it is more difficult to implement. The EU requirement is much easier to fulfill, and some experiments on nematode survival rates and storage temperatures have shown that this requirement can also achieve a fairly high level of safety. Japan does not have such a requirement, and there are many people who prefer raw food, so the number of cases of parasitic infection from eating raw fish in Japan is much higher than in Europe and the United States.

Some people prefer 'fresh' raw fish because freezing destroys the flavor and texture of the fish. Without heating or freezing, you have to rely on the naked eye to recognize the parasites and remove them manually. The good thing is that parasites are mostly bigger than bacteria and can be seen with the naked eye. It is also possible to get rid of them if the meat is sliced very thinly and inspected very carefully. However, whether it is 'guaranteed safe' or not also depends on how many parasites are in the fish itself, and whether the inspection removes them completely.