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HIV falls under the class of lentiviruses, which are a type of retrovirus that shows a long "lag time" between the time of infection and the rise of first symptoms. Other lentiviruses include SIV (simian immunodeficiency virus) and FIV (feline immunodeficiency virus -- anyone who has a cat with this knows it's misery).

The envelope of the viral particle has proteins gp41* and gp120*, which are recognition particles that bind to specific receptors on certain types of cells. It should be noted that not all CD4+ T-cells are infected, and not all T-cells that do NOT carry the CD4 surface protein are infected, either. The variance lies in the coreceptors CXCR4 and CCR5, which allow proper "docking" of the virus and encourage its entry into the cell. Without these coreceptors present, the T-cell cannot take up the virus; thus, there are people who can carry and transmit the human immunodeficiency virus without ever showing any symptoms themselves.

Incidentally, it should also be noted that HIV can also infect macrophages and other lymphocytes besides T-cells.

The genome is actually quite impressive. Through alternate splicing, frameshift induction, and overlapping genes, the virus has managed to squeeze 7 different known genes, resulting in at least a dozen or so mature proteins, into about 9.8 kilobases of RNA.

The genome looks something like so:

                                       | Vif  |       | U |
     |    Gag     |                          | R |      |         Env         |
LTR-------------------------------------------------------------------------------LTR
               |       Pol             |       | Tat |              |T|      |Nef|
                                                  |Re|              |Rev|

Where:
  • Gag = group antigen = matrix, nucleocapsid, capsid proteins
  • Pol = polymerase = protease, integrase, reverse transcriptase, Rnase H
  • Env = envelope proteins = gp41, gp120
  • Tat + T = transcriptional activator = regulatory gene this gene is a product of the Tat and T sections being spliced together
  • Rev +Re = RNA binding protein = shifts gene production this gene is a product of splicing the Re and Rev sections together
  • Nef = negative factor = regulation of cellular activities
  • R = Vpr = protein incorporated in virion = allows entry into nondividing cells
  • U = Vpu = membrane phosphoprotein = downmodulates CD4, promotes virion release
  • Vif = essential for growth only in certain cell lines
  • LTR = long terminal repeat = required for delineation of viral genome when incorporated into host genome; can allow repeating of viral genome within host by linking LTRs

Due to the error-prone viral RTase, HIV is very prone to mutation. (The mutation rate is approximately 1 in 106 bases.) Taking this and the rate of viral production into account, it seems that it would be very easy for the virus to mutate and become resistant to any form of treatment used. For this reason, HAART (highly active anti-retroviral therapy), which usually involves a "cocktail" of two protease inhibitors and a transcription inhibitor, was developed. The probability of developing mutations sufficient to render all three drugs is 1 in 1018, or very small. In fact, HAART is sufficient to maintain low enough serum levels of HIV particles for a patient to survive for several years. However, the entire regimen must be carefully maintained, as the virus lie latent and will begin replicating again once treatment is stopped.
* -- gp stands for gene product.

sources:
http://www.aegis.org
http://hivinsite.ucsf.edu/InSite.jsp