Tapeworm infection is caused by ingesting food or water contaminated with tapeworm eggs or larvae. If you ingest certain tapeworm eggs, they can migrate outside your intestines and form larval cysts in body tissues and organs (invasive infection). If you ingest tapeworm larvae, however, they develop into adult tapeworms in your intestines.
Tapeworm infections are most commonly seen in developing countries and are rare in the UK. In the UK, a tapeworm infection usually occurs when you eat raw contaminated pork, beef or freshwater fish.