There is a big potential problem here. It involves something that's small, pink, and crawls.
HINT: You're not likely to chuck it under the chin and go, "Ooh, CUTE Baby Snookums."
You got it, pal. Worms.
Eating raw fish can result in anisakiasis, an infection caused by an infestation of Anisakis worm larvae.
Now, don't panic, it's not that bad. They're not BIG worms. Not like one of those tapeworms, which can be a foot or two in length and take up more room in your abdominal cavity than you do.
No, these are LITTLE worms. They grow up to a mere one inch in length.
This is not comforting you, I can tell. Sorry, I'm doing the best I can.
If you're lucky, the worms will wind up in your stomach, where the chief symptom is generally a sudden attack of intolerable pain. It starts within 12 hours after eating the affected fish and continues for two or three days, until the worms expire.
If you're not as fortunate, the larvae head down to your intestines, where they can take up permanent residence.
You could think of them as low-maintenance pets. I mean, you don't have to walk them. To be considerate, every once in a while you could sort of jiggle, so they get some exercise. As for eating--well, I guess they take care of that on their own.
Maybe you don't want to think about it.
Cases of anisakiasis turn up from time to time in Japan and the Netherlands, where raw fish eating is common. Here the disorder is often misdiagnosed as appendicitis, peptic ulcer, or stomach cancer.
The only treatment is to poke a tube down your craw and remove the larvae one by one. The only preventive measure is to cook the fish or else freeze it at least three days.
(Mercifully, many Japanese restaurants purchase squid and whatnot frozen. Shrimp, eel, and octopus are often cooked.)
Some of the assassins who run sushi bars will tell you they can check for worms by "candling," holding the fish up to the light and cutting out the larvae before slapping what's left on your platter.
The Centers for Disease Control, however, say the efficacy of this method is on a par with rain dancing.
http://www.straightdope.com/classics/a2_060.html