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. Take a look at the life cycle of a parasitic worm. The parasite matures and reproduces in marine mammals. Next, the parasite eggs pass with the feces into the water, and hatch into larvae. Then, small water creatures such as crustaceans swallow larvae.
Fish eat small water creatures and thus become infected.
Finally, marine mammals eat fish, and then the whole cycle starts again. Humans interrupt this cycle by eating fish. There are two types of parasitic worms that can infect humans. One type of infection, called anisakiasis, is caused by ingesting the larvae of several types of roundworm. Symptoms include abdominal problems and fever, and may resemble appendicitis and intestinal obstruction.