Raw fish may contain anisakis parasites. These cause digestive disorders, stomach pain, nausea and, in 25% of cases, diarrhea and vomiting. Symptoms arise a few hours after eating.
The eggs of this parasite hatch in water. Larvae are swallowed by crustaceans, which are in turn devoured by fish. If left untreated anisakis parasites can paralyze large mammals.
Cooking over 130° Fahrenheit (60° centigrade), and freezing both destroy anisakis parasites.
If you experience the symptoms described above (many people mistake them for appendicitis) consult a doctor immediately.