Anisakis, a roundworm, typically does not grow to more than two centimeters long and about the width of a human hair, but the parasite may cause big problems for kosher fish lovers.

Some charedi rabbonim in Israel as well as a number of their American counterparts have ruled that the fish that the parasites inhabit, including wild salmon, flounder and halibut, can no longer be considered kosher. The Orthodox Union, the largest kashrut certifying organization in the world, does not believe such a ban is required or even necessary.

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