I love ceviche—raw fish marinated in citrus juice. Does the dish pose a threat from bacteria and parasites? —J.K., New Paltz, N.Y.

It could, but proper preparation can minimize the risk. Ceviche is made with raw fish or shellfish and lime or other citrus juice. Over time, the citrus "cooks" the fish by changing its molecular structure, similar to the effect of heat. But the acidity of citrus does not kill bacteria as effectively as heat, especially if you don't marinate the dish long enough. And to kill parasites without heat you have to freeze fish. If you're making ceviche, buy frozen fish or freeze fish at -4º F (-20º C) or lower for seven days. Keep fish cold until you use it, cut it into small pieces so the juice can penetrate the flesh better, and let it marinate in the refrigerator for the amount of time recommended in your recipe. When you eat out, ask your server whether the fish used in the ceviche was frozen appropriately. Children, seniors, pregnant women, and people with compromised immunity should avoid ceviche entirely.

Read more about fish risks and learn how to protect yourself from foodborne illness.