A tapeworm cannot live on its own; it needs a host's body to survive. The tapeworm thrived inside its host's intestines.
Pro Tip 2/3
Negative Connotation
'Tapeworm' implies something unwelcome and harmful, often used metaphorically to describe exploitative situations. That company is a tapeworm, draining its clients' finances.
Pro Tip 3/3
Transmission Vector
Tapeworms are usually transmitted through undercooked or contaminated food. He got a tapeworm from eating undercooked meat.