Diphyllobothriasis is an infection from a fish tapeworm.

The fish tapeworm (Diphyllobothrium latum), is the largest parasite that infects humans. Humans become infected when they eat raw or undercooked freshwater fish that contain tapeworm cysts.

The infection is seen in many areas where humans eat uncooked or undercooked fish from rivers or lakes. Diphyllobothriasis is seen in Eastern Europe, North and South America, African countries in which freshwater fish are eaten, and in some Asian countries.

After a person has eaten infected fish, the larva begin to grow in the intestine. They are fully grown in 3 - 6 weeks. The adult worm, which is segmented, may reach a length of 30 feet. Eggs are formed in each segment of the worm and are passed in the stool. Occasionally, parts of the worm may also be passed in the stool.

Fish tapeworm infection may lead to vitamin B12 deficiency and megaloblastic anemia.