Vibrio vulnificus is a harmful bacterium that lives in warm ocean waters where oysters thrive. When people with certain medical conditions consume contaminated oysters, they may die or become seriously ill. To avoid consuming or serving infected seafood, you should only buy from reputable suppliers and check each oyster before purchase to make sure it's alive.
Observe whether the oysters are open or closed. Most closed oysters are alive. Lightly tap an open shell – a live oyster will immediately snap shut. If the shell remains open, the oyster is dead and shouldn't be purchased or used.
Hold an oyster in your hand. The shell should feel full. If the shell doesn't feel heavy, tap it lightly against another oyster's shell. A hollow sound indicates a dead oyster. If the oyster is alive, you will hear a solid sound.
Smell the oyster at the market and again during food preparation. Live oysters should smell sweet, fresh and briny. Dead oysters have a strong fishy smell.