HIV is a cytopathic retrovirus that kills infected cells. The viral genes are carried as a single-stranded RNA molecule within the viral particle. Following infection, the virus selectively attacks host cells involved in immune function, primarily CD4 T lymphocytes. Within the host cell, the RNA template is reverse-transcribed into DNA (using the enzyme reverse transcriptase), which then becomes permanently integrated into the host's genome. Once integrated, retroviral DNA may lie dormant or be activity-transcribed and translated to produce virally encoded proteins and new HIV virions. Activation of viral protein precursors into the functional enzymes required for virion infectivity is carried out by HIV protease.

The HIV virion is composed of a central RNA molecule and reverse transcriptase protein surrounded by a core protein encased by a lipid bilayer envelope. Two virally encoded transmembrane proteins, gp41 and gp120, play a critical role in the recognition and attachment of HIV virions to receptors on host CD4 lymphocytes. As a result of infection, immunologic abnormalities eventually occur, including lymphopenia, qualitative CD4 T-lymphocyte function defects, and autoimmune phenomena. Profound defects in cellular immunity ultimately result in a variety of opportunistic infections and neoplasms.

Antiretroviral therapy is directed at reducing levels of HIV RNA by interfering with the activity of the enzyme's reverse transcriptase (nucleoside and nonnucleoside reverse transcriptase inhibitors), and HIV protease (protease inhibitors). Although use of antiretroviral agents has had a significant impact on progression of disease, long-term inhibition of viral replication has remained elusive, due to a combination of factors including drug toxicity and the emergence of mutant, drug resistant strains of HIV.

Transmission of HIV has been shown to occur via semen, vaginal secretions, blood or blood products, breast milk, and transplacental transmission in utero. HIV has also been isolated from saliva, urine, cerebrospinal fluid, brain, tears, alveolar fluid, synovial fluid, and amniotic fluid. Transmission has never been documented to occur by casual contact, although there is one reported case of possible salivary transmission by deep kissing from an HIV-infected male with bleeding gums to a female partner. HIV is a very labile virus and is easily neutralized by heat or common disinfecting agents, such as Lysol, a 1:10 solution of household bleach, 0.3% hydrogen peroxide, 35% isopropyl alcohol, or 50 percent ethanol.

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