HIV-2 is one of the two recognized types of human immunodeficiency virus, alongside HIV-1. Like HIV-1, it is a Lentivirus whose infection can lead to AIDS. These type names identify related viruses; they do not name successive stages of one infection.[ref]
Human Immunodeficiency Virus in the NIH HIV Glossary. definition, two HIV types and advanced disease stage
[ref]A landmark 1986 report by F. Clavel and colleagues described a distinct human retrovirus recovered from two patients with AIDS from western Africa. The researchers found biological and morphological similarities to the virus then called LAV/HTLV-III, while some antigens differed. The combination made the classification question experimentally meaningful: similar appearance and disease association did not establish that the agents were identical.[ref]
Isolation of a new human retrovirus from West African patients with AIDS by F. Clavel and others. Science 233, 343–346, Abstract: isolation and partial characterization
The logic can be illustrated with an abstract identification problem. Suppose two virus preparations share an observed structural feature, while one tested antigen differs. The shared observation supplies evidence of similarity; the differing observation supplies a way to distinguish the preparations. Neither one measurement alone establishes all their evolutionary relationships. A scientific classification combines such comparisons with further molecular evidence.
The shared reverse transcription strategy explains the placement of HIV-2 within Retroviridae, whereas distinguishing it from HIV-1 requires information finer than that family-level mechanism. A broad replication category and a type-specific identification answer different biological questions.
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