Classification and structure

The etiological agent of AIDS, human immunodeficiency virus type 1 (HIV-1), now endemic worldwide, was discovered in 1983. Two years later, a second virus (HIV-2) that caused the same disease but was significantly different from HIV-1 serologically and molecularly to be considered a separate, bur related, virus was discovered in West Africa. Between 1985 and 1989, several nonhuman primates were shown to harbor similar viruses, some of which cause disease in the natural host and some of which appear to be quite harmless. All of these viruses have an RNA genome and replicate via a DNA intermediate (a 'provirus') by means of viral RNA-directed DNA polymerase, more commonly called reverse transcriptase. It is this 'backward' transfer of genetic information from RNA to DNA which classifies these viruses as retroviruses.

Each virion measures approximately 120 nm in diameter and has condensed cylindrical core surrounded by a lipid membrane. The envelope 'spike' glycoprotein is embedded in and protrudes from this membrane. The interrelationship of genomic RNA, core proteins and surrounding envelopes is more clearly demonstrated schematically (Figure 1) and more readily understood by considering the genomic organization.

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