Host immunity
Hosts can become immune to infection by a cestode if the lining, the mucosa, of the gut is damaged. This exposes the hosts immune system to cestode antigens, enabling the host to mount an antibody defence. Host antibodies can kill or limit cestode infection by damaging their digestive enzymes, which reduces their ability to feed and therefore to grow and to reproduce; by binding to their bodies; and by neutralising toxins that they produce. When cestodes feed passively in the gut, they do not provoke an antibody reaction. Evolution and phylogeny
Fossil history
Parasite fossils are rare, but recognizable clusters of cestode eggs, some with an operculum (lid) indicating that they had not erupted, one with a developing larva, have been discovered in fossil shark coprolites dating to the Permian, some 270 million years ago.The fossil Rugosusivitta, which was found in China at base of the Cambrian deposits in Yunnan  just above the Ediacaran-Cambrian border, has great similarities to present day Cestodians. If correct, this would be the earliest example of a Platyzoan and also one of the earliest bilaterian body-fossils and might thus provide an insight to the living mode of Cestodians before they became specialized parasites. External
The position of the Cestoda within the Platyhelminthes and other Spiralian phyla based on genomic analysis is shown in the phylogenetic tree. The non-parasitic flatworms, traditionally grouped as the "Turbellaria", are paraphyletic, as the parasitic Neodermata including the Cestoda arose within that grouping. The approximate times when major groups first appeared is shown in millions of years ago.
When television presenter Michael Mosley deliberately infected himself with tapeworms he gained weight due to increased appetite. Dieters still sometimes risk intentional infection, evidenced by a 2013 warning on American television. Notes
References
Further reading

Merck Manual of Medication Information, Second Home Edition, Online Version, Tapeworm Infection 2005
Mayo Clinic Website on infectious diseases, Mayo Clinic - Tapeworm Infection, 2006
Medline Plus - Taeniasis (tapeworm infection)
University of South Carolina - School of Medicine - Cestodes (tapeworms)