Sushi, a popular Japanese food which often contains raw fish, has been the question of some inquiries about food safety. Some people worry about whether it is safe to eat, because raw foods typically carry a higher risk of food borne illness. Some species of fish are also contaminated with mercury and various chemicals, making people wonder about how much fish they should be eating, let alone whether the fish is raw or cooked. There are some things that make sushi safe to eat under certain conditions and not in others, so it's hard to draw a hard and fast line about how safe this food is.
Some people confuse sushi, a dish made with vinegared rice and other ingredients which sometimes include raw fish, with sashimi, a dish which contains raw fish alone. In some cases, people ensure their safety by only eating sushi that does not contain raw fish. Vegetarian rolls tends to be quite safe, as does sushi with fish which has been cured or cooked to eliminate potential sources of parasitic infection, along with bacteria which might have gathered on the fish.
In the case of sashimi and sushi made with raw fish, two factors go into making sushi safe: the type of fish, and how the fish is handled. As a general rule, freshwater fish species are not safe to eat raw, because they often contain parasites which can only be eliminated by cooking. Ocean species are less likely to have parasites, especially if they are handled properly, which brings up the second factor. The best fish are processed rapidly at sea and then frozen. Rapid processing removes the intestines before they have a chance to burst and allow bacteria and parasites into the body cavity. Freezing kills any parasites which might have lingered, rendering the fish safe to use.
Some nations have a sushi grade for fish labeling which is used on fish which has been well handled and quickly flash-frozen, indicating that this fish can safely be used in sushi. If it is kept cold until use and handled in a clean kitchen by a cook with clean hands, this fish is, as a general rule, very safe from a food-borne illness perspective, although cooked fish is generally safe. However, fish like tuna are very high in mercury, so people who eat lots of sushi may be at risk for mercury poisoning. For this reason, people may choose to avoid tuna, sea bass, and swordfish, as these fish can contain high levels of mercury.
There are certain people for whom sushi is not safe. People with compromised immune systems should not eat it, because if there are lingering parasites or bacteria, they could easily overwhelm the body while it was weak. It is also not recommended for pregnant women, both because of the increased risk of infection for the baby, and because consuming fish tainted with mercury could potentially lead to developmental delays for the growing baby.
If you want to be sure you're eating safe sushi, only eat dishes prepared by reputable cooks in commercial kitchens, where cooks have access to freezers which can get extremely cold, along with meticulously clean facilities. If you prepare this dish at home, buy sushi-grade fish and use it promptly, because your freezer probably does not get cold enough to keep the fish safe. Take care to wash your hands and all your kitchen surfaces before and after making sushi to keep bacteria to a minimum. Never eat sushi which has been sitting at room temperature.