The problem occurs because raw mollusks sometimes carry bacteria called Vibrios that may multiply after the shellfish are caught, even with refrigeration. These bacteria are completely killed when the shellfish are thoroughly cooked, removing all danger od the bacteria causing food poisoning. But if the mollusks are eaten raw or partially cooked, the bacteria remain alive and may make you very sick. You should be evaluated by your physician if you experience any of these symptoms within five days after eating raw or undercooked molluscan shellfish. Infection with some Vibrios can cause blood poisoning, a condition that has resulted in death in up to half the people who get it.
Death usually occurs within two days after the illness starts.
Certain viruses known as Norwalk viruses also could contaminate oysters, clams and mussels and cause severe diarrhea in those who eat them. Here again, thorough cooking kills the virus. Mollusks usually live where rivers and seas meet. Because many cities are located near those places, the waters are more likely to be polluted than offshore waters. For this reason, shellfish harvesting is prohibited in areas contaminated by sewage.