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. To enforces this, these areas are patrolled by state health and fishery agencies. Mollusks feed by filtering water through their systems, so they are more likely to pick up and store bacteria or viruses from the water, including those that van cause illness in humans. If you eat mollusks raw, you eat the live viruses and bacteria too.