Parasitism

Parasitism is an interaction in which a parasite obtains resources from a host, reducing some component of the host's performance. Its demographic effects depend on exposure, within-host processes and transmission opportunities.

A parasitic worm; the term groups several distinct animal lineages rather than one taxon.

A genus of parasitic roundworms, including Trichinella spiralis.^[ref]

A host in which a parasite reaches sexual maturity.

A host carrying developmental stages without the sexually mature adult stage of the parasite.

Historical collective name for internal animal parasites, especially parasitic worms. It is not a single modern clade.

Evolution of host-independent growth from a host-dependent lineage. It requires replacing host-provided functions, not merely losing virulence or host-invasion genes.

An organism whose life cycle requires a host; survival of a transmission stage outside a host does not establish host-independent reproduction.

Dependence on host-provided resources, physical conditions, signals or transmission opportunities; the required functions vary greatly among parasite lineages.

A parasitoid develops on or in a host and eventually kills it. Host-parasitoid transmission therefore has different offspring accounting from ordinary predation or a nonlethal parasite-burden model.

A biological host provides the environment or resources used by a parasite or other associated organism.

A parasite is an organism benefiting from parasitism. Microparasite and macroparasite distinguish useful transmission-model descriptions of within-host multiplication and distributed parasite burden.

A corallicolid is a member of a coral-associated apicomplexan lineage. Retention of chlorophyll-biosynthesis genes without photosystem genes illustrates why a partial pigment pathway is not evidence for complete photosynthesis; its physiological role requires further testing.

Cryptosporidium parvum is an apicomplexan parasite lacking an apicoplast. Its organelle and metabolic inventory provides a comparison for lineages retaining a plastid.

Plasmodium falciparum is an apicomplexan parasite with stage-dependent metabolic requirements. Blood-stage chemical-rescue experiments identify an essential apicoplast output under defined culture conditions.

An apicomplexan parasite belongs to an evolutionary lineage containing organisms such as Plasmodium and Cryptosporidium. Plastid retention and metabolic requirements vary among lineages and life-cycle stages.

A macroparasite is typically modelled using the number of parasites carried by each host. Adult parasitic worms illustrate why within-host parasite burden, developmental stages, mating and host exposure heterogeneity matter beyond an infected/not-infected classification.

The within-host parasite burden is the number of parasites, or a specified parasite-load measure, carried by one host. A population's mean burden and its infected-host prevalence are different summaries of the burden distribution.

A microparasite is typically modelled as multiplying rapidly within a host, so infection status rather than the exact organism count is the first between-host state variable. SIS model, SIR model and SEIR model descriptions are examples. Additional within-host structure can still be required.