If you're a strict orthodox Jew, worms aren't kosher, but worms in fish are. Why? Because ancient scholars believed in "spontaneous generation," so the worms were thought to be creatures that didn't "crawl on the ground." Now that we know there's no such thing as spontaneous generation (apart from all the insanely heavy objects I don't remember putting in my suitcase when I packed for this book-tour), can fish-worms still be kosher? Apparently so, if we're to believe Yeshiva World, which proposes resolving this conundrum by simply rejecting the science that disproves spontaneous generation. All hail the "la la la I can't hear you" school of theology.
This article proposes that Halachah rejects migration to the flesh, even when identical intestinal worms are present. Rather, we presume that flesh worms are internally generated, even if we are uncertain of the biological mechanism by which this occurs. This approach simply resolves all difficulties but one – that scientists say that non-invasive worm generation is impossible. In this matter, we reject the evidence of experimentation in favor of the word of Chazal, whose tradition-based biological knowledge exceeded human experimental abilities.
(Image: Red Wiggler Worms, a Creative Commons Attribution (2.0) image from wheatfields's photostream)