Raw oysters are a delicacy for many but potentially deadly for people with certain health problems.

That's because raw or undercooked oysters may contain vibrio vulnificus, bacteria that can cause severe food-borne illness.

(ENLARGE)

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Those with liver damage, whether from heavy drinking or liver disease, are especially at risk. If the bacteria cause a bloodsteam infection, about half of those with liver conditions will die, according to the Centers for Disease Control and Prevention. People with weakened immune systems, such as those with diabetes or cancer, also are at risk.

The bacteria are rare, with about 40 diagnosed cases of oyster-related vibrio infection in Americans annually. Four people died from vibrio infections this year in Georgia after eating raw seafood, according to the state Division of Public Health. Shellfish are most likely to be contaminated between April and November, when waters in the Gulf of Mexico and the south Atlantic are warm, according to the University of Georgia Marine Extension Service. More than 85 percent of infections occur between May and October, according to the CDC.

Vibrio infections were one of the few food-borne illnesses to rise significantly in the last decade, up 41 percent, according to the CDC. You can't tell if seafood contains the bacteria by smell, taste or appearance.

If you have any of the following conditions, you shouldn't eat raw or undercooked shellfish, medical experts say:

• Alcohol abuse

• Liver disease

• Cancer, including lymphoma, leukemia and Hodgkin's disease

• Diabetes

• Stomach disorders, including taking antacids

• Inflammatory bowel disease

• Kidney disease

• AIDS/HIV

• Abnormal iron metabolism (hemachromatosis, hemosiderosis); hemolytic anemia

• Steroid dependency, as used for conditions such as emphysema, chronic obstructive pulmonary disease and others

• For those with weakened immune systems who want to eat oysters, the CDC recommends these methods to ensure the shellfish is thoroughly cooked. Since heat kills the bacteria, thoroughly cooked oysters are OK.

• For oysters in the shell: Boil until the shells open, and continue boiling for five more minutes. Or steam until the shells open, and then continue cooking for nine more minutes. Do not eat those shellfish that do not open during cooking.

• For shucked oysters: Boil at least three minutes, or fry in oil at least 10 minutes at 375 degrees.

• The Interstate Shellfish Sanitation Commission recommends these additional cooking methods for shucked oysters: Bake for 10 minutes at 450 degrees, or broil three inches from the heating element for three minutes.

Buying oysters

• Buy only from stores with a current business license.

• Ask to see the certified shipper's tag that is required for all legally harvested shellfish. The tags indicate the oysters were harvested from approved waters. Don't buy oysters without a tag; they could come from polluted waters.

• Check to make sure oysters in shell are alive; shells should be closed or slightly open. Tap on the shell if it's open; if it closes, the oyster is alive. If the shell doesn't close, throw away the oyster and do not eat it.

• Shucked oysters packed in containers are meant for cooking and should not be eaten raw.

Storing oysters

• Live oysters (in the shell) should be stored in the refrigerator and loosely covered with clean, damp paper towels or cloths to prevent them from drying out. Do not store in airtight containers or on ice because the oysters could die. Don't store below raw meat that could drip juices onto the oysters and contaminate them.

• Live oysters are best eaten within one to two days after purchase, although they can live for up to a week or longer.

• Shucked oysters should be eaten by the date on their container, or within four to seven days of purchase.

• Leftover cooked shellfish should be refrigerated and eaten within one to two days.

Sources: University of Georgia Marine Extension Service, Interstate Shellfish Sanitation Commission, Centers for Disease Control and Prevention, Georgia Division of Public Health

Shucking oysters

Wash the outside of the shell and scrub with a stiff brush to remove dirt that could get on the oyster. Use a shucking glove or thick towel to hold the oyster to guard against cuts from the knife or oyster shell. Hold oyster so the flat side of the shell is on top, the deeper cup on bottom. Using an oyster knife (available at many seafood counters), place the knife between the shell halves near the hinge and cut around the shell to separate the muscle holding it together. Be careful not to spill the liquid from the shell. Pry the shell apart. Carefully run the knife across the top of the oyster to release it from the top shell, then repeat for the bottom shell. Remove the top shell. The oyster is ready for eating or cooking.