The growing popularity of sushi in the West has led doctors to warn of ingesting potentially deadly parasites.
Raw and undercooked fish can contain tiny worms that bury into the human gut if eaten, causing a condition known as Anisakiasis, which can lead to severe stomach pain, vomiting and fever.
In severe cases the parasites can cause an anaphylactic reaction, with the worms causing potentially fatal respiratory failure and a fast heartbeat.
In the past most cases have typically been found in Japan, where raw fish is a common dish.
But the rise in popularity of sushi in the West has led to cases in hospitals across Europe.
One recent incident saw a previously healthy 32-year-old Portuguese man left in agony for a week after eating raw fish from a Japanese restaurant. He was also vomiting and had a fever.
After going to hospital in Lisbon, tests showed a mild inflammation and a tenderness in the area below his ribs.
But it was only when the man revealed he had recently eaten sushi that doctors suspected he might have Anisakiasis.
They put an endoscope – a long tube with a camera – down his throat and into the stomach, where the larvae of a worm-like parasite was found firmly attached to the gut lining.
The larvae was removed with a special kind of net "and the patient's symptoms resolved immediately," the medical team said in a British Medical Journal Case Report.
They noted that most cases of Anisakiasis to date have been reported in Japan, but warned "it has been increasingly recognised in Western countries".
Dr Joana Carmo, a gastroenterologist at the Hospital of Egas Moniz in Lisbon, said: "Owing to changes in food habits, Anisakiasis is a growing disease in Western countries, which should be suspected in patients with a history of ingestion of raw or uncooked fish.
"Patients can have allergic symptoms ... Gastrointestinal symptoms include abdominal pain, nausea and vomiting and complications like digestive bleeding, bowel obstruction, perforation and peritonitis can also arise."
According to the Centers for Disease Control and Prevention, some people may experience a tingling sensation while eating infected raw or undercooked fish – said to actually be the worm moving in the mouth or throat.