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Feline immunodeficiency virus is a Lentivirus of cats, classified within Orthoretrovirinae. Its connection to immune dysfunction made it a useful subject for studying persistent retrovirus infection in a host distinct from the human host of human immunodeficiency virus.^[ref]

The 1987 report by N. C. Pedersen and colleagues described isolation of a T-cell-associated virus from a cattery with an immunodeficiency-like syndrome. The animals had tested seronegative for feline leukemia virus, and the newly isolated agent was antigenically distinct from human immunodeficiency virus. These comparisons addressed whether an already recognized agent accounted for the observations; a shared disease description alone would not establish viral identity.^[ref]

The same report found antibodies to the new agent in 10 of 25 cats showing signs of ill health and in 1 of 18 apparently normal cats. Those proportions are $40\%$ and approximately $5.6\%$. Their difference motivates investigating an association, while the positive apparently normal animal shows why infection and observed illness cannot be treated as identical categories.^[ref]

These are counts from selected groups in one investigation, not a population-wide prevalence estimate. Serology measures evidence of a host response under an assay's criteria; establishing the complete causal sequence of a disease calls for further observations.

Human immunodeficiency virus is the name used for two related retroviruses, HIV-1 and HIV-2, whose infection can lead to AIDS. The virus and the disease stage are distinct: AIDS is the most advanced stage of human immunodeficiency virus infection, rather than the name of every infection.^[ref]

Both belong to Lentivirus, within Orthoretrovirinae and Retroviridae. Their replication connects an RNA genome, a DNA intermediate formed by reverse transcription, and an integrated provirus. In primate lentiviruses, entry involves CD4 and a chemokine receptor; identifying a cell receptor and measuring subsequent virus production are distinct experimental questions.^[ref][ref]

A constructed measurement example distinguishes a molecular quantity from a biological outcome. Suppose an assay reports 100 viral RNA copies in an analysed equivalent of one microlitre of plasma. The corresponding viral load is $100\times 1000=100,000$ RNA copies per millilitre, assuming the assay's stated calibration and volume correction. That count is not a direct count of infectious virions, and it does not by itself specify a person's disease stage. The measured material, normalization and biological question must all be stated.

Orthoretrovirinae is the subfamily of Retroviridae containing Lentivirus and several other retroviral genera. It includes human immunodeficiency virus and feline immunodeficiency virus. The family placement describes a replication strategy; it does not imply one host range or one disease outcome for all members.^[ref]

A typical virion packages two copies of its RNA genome. After entry and release of the viral core into a host cell, reverse transcription produces DNA that can become an integrated provirus. Proteolytic processing of a viral polyprotein also releases functional components during maturation.^[ref][ref]

An invented counting example clarifies the two-copy description. If each packaged RNA molecule has 10,000 nucleotides, the pair contains 20,000 RNA nucleotides in total. The length of one genome copy is still 10,000 nucleotides. A corresponding 10,000-base-pair double-stranded DNA molecule contains 20,000 DNA nucleotides, but its length is conventionally reported in base pairs. Copy count, nucleotide count and base-pair length answer different questions.

The subfamily's genera are distinguished in part by phylogenetic analysis of conserved reverse-transcriptase regions. A shared enzyme mechanism can coexist with differences in accessory proteins, host distribution and cellular targets. Observing the shared mechanism therefore provides a classification clue while leaving those other biological properties to be determined.^[ref]

Retroviridae is a taxonomic family of enveloped viruses whose replication involves reverse transcription and integration of the resulting DNA into host-cell DNA. The integrated form is a provirus, from which host machinery can make viral RNA. Thus the replication cycle links RNA and DNA forms rather than remaining entirely in one nucleic-acid form.^[ref]

A virion contains a viral envelope, an internal core, genomic nucleic acid and replication enzymes. Orthoretrovirinae package a dimer of single-stranded RNA. In Spumaretrovirinae, reverse transcription can already be partly completed in the particle, so a family-wide description should not require every virion to contain only RNA.^[ref]

HIV-1, in the genus Lentivirus, is one member of this broader family. The family name therefore supplies a classification shared by multiple genera, rather than another name for HIV-1 alone.

The RNA-to-DNA-to-RNA cycle also clarifies measurement. A test detecting a proviral DNA sequence addresses a different molecular form from a test detecting packaged RNA. Demonstrating one intermediate can support the proposed replication pathway, but its abundance alone does not count every completed infectious particle. The assay must identify which stage and material it measures.

Spumaretrovirinae is a subfamily of Retroviridae that includes the foamy viruses. The name refers to a characteristic foamy appearance that many isolates produce in cell culture. That visible effect is one observation within a classification also supported by molecular features and phylogenetic analysis.^[ref]

A notable feature concerns the timing of reverse transcription. Released virions can already contain substantial reverse-transcribed DNA. In Orthoretrovirinae, the usual packaged genome is instead two RNA copies and reverse transcription follows entry into a new host cell. Both groups use a DNA stage in their replication cycles.^[ref][ref]

The ICTV account also describes cytoplasmic capsid assembly and a genome of roughly 11.6 kilobases for a typical spumavirus. Its viral enzyme precursor is expressed from a spliced transcript, rather than the same fused precursor arrangement emphasized for Orthoretrovirinae.^[ref]

The timing distinction gives a useful experimental question: at which point in the cycle is a particular nucleic-acid form present? Detecting DNA in released particles and detecting newly made DNA after entry describe different stages. A single positive measurement, without identifying the sampled stage and material, cannot reconstruct when conversion occurred. Molecular form, location and sequence in time are complementary evidence.