Parasitism and symbiosis are Nature's extreme forms of commensalism (physical association). A parasitic species promotes its own interests entirely to the detriment of the other; symbiosis refers to the much less common state in which both organisms obtain some benefit from the association.
Imaginary parasites of human beings are featured in many effective sf Horror stories, often linked to the idea of vampirism (although classical Vampires might better be regarded as predators than as parasites). Stories dealing with Life on Other Worlds often feature parasites which are exaggerated versions of earthly creatures. Those insects which lay their eggs in living hosts are popular models; they feature in A E van Vogt's "Discord in Scarlet" (December 1939 Astounding; incorporated in The Voyage of the Space Beagle, fixup 1950), the film Alien (1979) and its sequels, and Octavia E Butler's "Bloodchild" (June 1984 Asimov's); the closely related notion of the mother killed by her internal young appears in Philip José Farmer's The Lovers (August 1952 Startling; exp 1961) and Gardner Dozois's Strangers (in New Dimensions IV, anth 1974, ed Robert Silverberg; exp 1978). Parasites leeching the "vital energy" of human beings are commonplace; when the parasites are internal rather than external this often involves the will of the victim being usurped, thus referring metaphorically to demonic possession as well as to vampirism. Early examples of this kind of story include J Maclaren Cobban's Master of His Fate (1890) and Arthur Conan Doyle's The Parasite (1895); the classic Pulp-magazine sf extrapolations are Eric Frank Russell's Sinister Barrier (March 1939 Unknown; 1943; rev 1948) and Robert A Heinlein's The Puppet Masters (September-November 1951 Galaxy; 1951; text restored 1990). Other stories in similar vein are Eric Frank Russell's "Vampire from the Void" (1939 Fantasy #2), Philip José Farmer "Strange Compulsion" (October 1953 Science-Fiction Plus; vt "The Captain's Daughter" in The Alley God, coll 1962), Frank R Crisp's The Ape of London (1959), Robert Silverberg's "Passengers" (in Orbit 4, anth 1968, ed Damon Knight), Jack Vance's second and third Durdane novels – The Brave Free Men (1973) and The Asutra (1974) – Colin Wilson's The Mind Parasites (1967) and The Space Vampires (1976), David Cronenberg's film The Parasite Murders (1974) and Damon Knight's CV (1985).
This frequent movement of the notion of parasitism from the context of the mundane to the quasisupernatural is in keeping with sf's habitual treatment of biological themes (see Biology). In concert with general trends relating to Aliens there was a dramatic change of emphasis in post-World War Two stories, in which apparently parasitic relationships are often revealed to be in fact symbiotic. Some stories are conscious ideological replies to earlier works – Ted White's By Furies Possessed (1970), which attacks the implicit xenophobia of The Puppet Masters, is a notable example. The concept of symbiosis had earlier been used in some ecological puzzle stories (see Ecology), notably Eric Frank Russell's "Symbiotica" (October 1943 Astounding) and an ironic story of defensive biological warfare, "Symbiosis" (14 June 1947 Collier's Weekly) by Will F Jenkins (Murray Leinster), but the quasisupernatural connotations it eventually took on were decisively opposed to metaphors of vampirism and possession. It became a central notion of the "ecological mysticism" (see also Gaia) displayed in such works as Sydney J van Scyoc's trilogy Daughters of the Sunstone (1982-1984; omni 1985). Explicit religious imagery comes to the fore in such stories of human/alien symbiosis as Clifford D Simak's Time and Again (October-December 1950 Galaxy as "Time Quarry"; 1951; vt First He Died 1953), Bob Shaw's The Palace of Eternity (1969) and Nicholas Yermakov's trilogy begun with Last Communion (1981) (see Simon Hawke).
Post-World War Two stories in which human and alien minds share a body usually see such relationships as potentially symbiotic; examples include Hal Clement's Needle (May-June 1949 Astounding; exp 1950; vt From Outer Space 1957) and its belated sequel Through the Eye of a Needle (1978), Algis Budrys's "Silent Brother" (February 1956 Astounding as by Paul Janvier), Brian W Aldiss's The Long Afternoon of Earth (February-December 1961 F&SF; fixup 1962; exp vt Hothouse 1962) with its intelligent morel-fungus symbiote, Brian M Stableford's Halcyon Drift series (1972-1975), Roger Zelazny's Doorways in the Sand (1976) and F Paul Wilson's Healer (1976). Even Christopher Evans's bleak mind-parasite story The Insider (1981) is sympathetic to the parasitic consciousness, as is Stephenie Meyer's The Host (2008), filmed as The Host (2013). The more ambivalent view of human/alien commensalism adopted in Octavia E Butler's Clay's Ark (1984) and related works, and in the first part of Dan Simmons's Hyperion (1989) – where alien "cruciforms" grant perpetual Regeneration from death, cruelly accompanied by gradual loss of Intelligence with each revival – cleverly exploits and undercuts this modern sensibility. Similarly, Seanan McGuire's Parasitology series opening with Parasite (2013) as by Mira Grant is actually narrated by one of the symbiotic tapeworms installed in humans for excellent medical reasons, though of course with unforeseen consequences such as a variation on the now-traditional Zombie apocalypse.
Merely tiresome parasites include the flesh-burrowing worms of planet Sergyar, mentioned in Lois McMaster Bujold's Mirror Dance (1994) and later novels of her Vorkosigan saga; these produce a harmless but disfiguring kind of elephantiasis. Still cruder are such tales as John Grant's and David Langford's – which posits human digestive tracts as symbionts which after long abuse literally rise up and revolt – and Stephen King's Dreamcatcher (2001) with its bowel-infesting Alien "shit-weasel" parasites; the latter was filmed as Dreamcatcher (2003).
This area of speculation is perhaps the most obvious example in sf of the utility of biological notions as metaphysical metaphors (see Metaphysics), and of the way that such metaphorical usage dominates the expression of biological notions in sf. [BS/DRL]
see also: Hive Minds; Oni; Paranoia; Supernatural Creatures.
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