All but amphilinids and gyrocotylids (which burrow through the gut or body wall to reach the coelom) are intestinal, though some life-cycle stages rest in muscle or other tissues. The definitive host is always a vertebrate but in nearly all cases, one or more intermediate hosts are involved in the lifecycle, typically arthropods or other vertebrates. Infections can be long-lasting; in humans, tapeworm infection may last as much as 30 years. No asexual phases occur in the lifecycle, as they do in other flatworms, but the lifecycle pattern has been a crucial criterion for assessing evolution among Platyhelminthes.Cestodes produce large numbers of eggs, but each one has a low probability of finding a host. To increase their chances, different species have adopted various strategies of egg release. In the Pseudophyllidea, many eggs are released in the brief period when their aquatic intermediate hosts are abundant (semelparity). In contrast, in the terrestrial Cyclophyllidea, proglottids are released steadily over a period of years, or as long as their host lives (iteroparity). Another strategy is to have very long-lived larvae; for example, in Echinococcus, the hydatid larvae can survive for ten years or more in humans and other vertebrate hosts, giving the tapeworm an exceptionally long time window in which to find another host.Many tapeworms have a two-phase lifecycle with two types of host. The adult Taenia saginata lives in the gut of a primate such as a human, its definitive host.
While accidental tapeworm infections in developed countries are quite rare, such infections are more likely to occur in countries with poor sanitation facilities or where food hygiene standards are low. History and culture
In ancient Greece, the comic playwright Aristophanes and philosopher Aristotle described the lumps that form during cysticercosis as "hailstones". In Medieval times, in The Canon of Medicine, completed in 1025, the Persian physician Avicenna recorded parasites including tapeworms. In the Early Modern period, Francesco Redi described and illustrated many parasites, and was the first to identify the cysts of Echinococcus granulosus seen in dogs and sheep as parasitic in origin; a century later, in 1760, Peter Simon Pallas correctly suggested that these were the larvae of tapeworms.Tapeworms have occasionally appeared in fiction. Peter Marren and Richard Mabey in Bugs Britannica write that Irvine Welshs sociopathic policeman in his 1998 novel Filth owns a talking tapeworm, which they call "the most attractive character in the novel"; it becomes the policemans alter ego and better self. Mira Grants 2013 novel Parasite envisages a world where peoples immune systems are maintained by genetically engineered tapeworms. Tapeworms are prominently mentioned in the System of a Down song "Needles": their inclusion within the song result in a lyrical dispute among band members.There are unproven claims that, around 1900, tapeworm eggs were marketed to the public as slimming tablets. A full-page coloured image, purportedly from a womens magazine of that period, reads "Fat: the enemy ... that is banished! How? With sanitized tape worms. Jar packed. No ill effects!"