The herringworm — Anisakis simplex and its relatives — is a marine roundworm (nematode) whose larvae infest sea fish such as herring, mackerel, cod, and squid. If humans eat the fish raw or undercooked — as sushi, sashimi, ceviche, or lightly cured preparations — the live larvae can burrow into the stomach or intestinal wall, causing anisakiasis: violent abdominal pain, nausea, and vomiting within hours. The larvae cannot complete their life cycle in humans and eventually die, but the allergic reactions they trigger can be severe. Prevention is straightforward: the EU requires freezing of fish intended for raw consumption (typically −20 °C for 24 hours), and thorough cooking kills the larvae outright.