The problem occurs because raw mollusks sometimes carry bacteria called Vibrio vulnificus 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 of the bacteria causing food poisoning. But if mollusks are eaten raw or partially cooked, the bacteria remain alive and may make you very sick. Symptoms include sudden chills, fever, nausea, vomiting, and stomach pain. Vibrio vulnificus can cause blood poisoning, a condition that could be fatal in up to half the people who get it.
Death usually occurs within two days.
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.