Flash freezing is practiced in most Sushi restaurants, can't find statistics on how prevelent it is. It does cut down on the number of living parasites. Other strategies include the carefull selection of what is called "Sushi Grade Fish" which supposedly carries fewer parasites. Most SW fish parasites can not be transmitted to humans. One that can is Anisakiasis which is a parasitic disease contracted from infected seafood which is eaten raw or marinated. This is a type of round worm which can be picked up from eating sashimi, sushi, and ceviche.
Symptoms are variable and can range from an acute syndrome with nausea and vomiting, abdominal pain or diarrhea within 12 hours of eating infected seafood to other problems which can last weeks to months. Anisakiasis can mimic other diseases, such as acute appendicitis, stomach tumors, and inflammatory bowel disease.
Fewer than 30 cases of illnesses resulting from the presence of parasites in sushi or sashimi were reported in the U.S. during 1986, and most were on the West Coast. More fish are infected -- and therefore more humans -- on the West Coast than the East because the primary hosts for the parasites are marine animals commonly found in the Pacific, such as seals, porpoises, sea lions and whales
It's important to remember that adequate freezing and/or cooking eliminates infection by the parasites. In commercial freezing, a temperature of -40 Ã"šÃ‚°F kills any parasite in 15 hours. In a home freezer, at 0Ã"šÃ‚° to 10Ã"šÃ‚°F, it can take up to five days to kill all the parasites, especially in large fish.