When certain infected marine mammals (such as whales or sea lions) defecate into the sea, eggs are released and become infective larvae while in the water. These larvae are ingested by crustations, which are then eaten by squid or fish. When humans eat raw or undercooked infected squid or fish, they ingest nematode larvae. Once inside the body, the larvae can invade the gastrointestinal tract. Eventually, the parasite dies and produces an inflamed mass in the esophagus, stomach, or intestine.
Some people experience an tingling sensation after or while eating raw or undercooked fish. This is actually the worm moving in the mouth or throat. These people can often extract the worm manually from their mouth or cough up the worm and prevent infection. Also, some people experience vomiting as a symptom and this can often expel the worm from the body.

Anisakiasis is most commonly found in areas where eating raw fish is popular, such as Japan. However, since eating sushi is becoming very common, there have been cases seen in the United States, Europe, South America, and other areas of the world. Anyone who eats undercooked or raw fish or squid is at risk.

A history of eating undercooked fish or squid is helpful. Diagnosis is generally made by endoscopy, radiography, or surgery if the worm has embedded.