These tiny, cosmopolitan tapeworms do little harm to the carnivores in which the adult develops, often in population sizes up to several tens of thousands. However, in their intermediate hosts, including humans, their larval stage produces a large cyst which may cause serious damage to the host, directly as a result of mechanical damage, or indirectly through leakage of its highly immunogenic contents. The cysts of one species are often mistaken for cancer, and are eventually fatal.