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In 1981, the first cases of symptomatic HIV (human immunodeficiency virus) infection were recognized as AIDS (acquired immunodeficiency syndrome). It has been established that HIV infection is caused by a virion belonging to the family Retroviridae that invades the CD4+ lymphocytes. These lymphocytes are the "helpers" that modulate the actions of other cellular components of the immune system. The eventual loss of CD4+ lymphocytes is the underlying pathophysiology that leads to AIDS. With the breakdown of the immune system, an environment for establishing chronic infections is created which can be the cause of death in the HIV-infected patient. AIDS is now the fifth leading cause of death among adults between the ages of 25 and 44.(1)
For a person to become infected with HIV, there must be direct contact by way of the blood stream or mucous membrane of a non-infected person with infected blood, semen, or vaginal secretions. Intimate sexual contact is the primary route of HIV transmission. HIV can also be transmitted from mother to child perinatally, through transfusions, IV drug abuse, or exposure to infected blood or blood products in the health care setting. There is evidence that has demonstrated a lack of HIV transmission through casual and household contact. The half-life of the virus is only six hours, but an individual with moderate to advanced infection may produce hundreds of billions of viral particles each day.
HIV is a retrovirus that carries its genetic information in RNA rather than in DNA. The process of infection includes virus entry, reverse transcription, integration, gene expression, assembly, budding, and maturation. The stages of HIV reproduction proceed as follows:
- HIV enters a CD4+ cell.
- HIV is a retrovirus, meaning that the genetic information is stored on single-stranded RNA instead of the double-stranded DNA found in most organisms. To replicate, HIV uses an enzyme known as reverse transcriptase to convert its RNA into DNA.
- HIV DNA enters the nucleus of the CD4+ cell and inserts itself into the cell's DNA. HIV DNA then instructs the cell to make copies of the original virus.
- New virus particles are assembled and leave the cell ready to infect other CD4+ cells.
A diagnosis of AIDS is made in individuals with the following criteria:
A CD4+ cell count of less than 200 cells per cubic millimeter (or CD4 T-lymphocyte percentage of total lymphocytes of less than 14%) and a laboratory-confirmed HIV infection; OR
A laboratory-confirmed HIV infection plus one of a list of additional clinical conditions that includes: candidiasis, invasive cervical cancer, cytomegalovirus disease, histoplasmosis, mycobacterium, recurrent pneumonia, salmonella septicemia, and wasting syndrome due to HIV.
Plasma HIV RNA levels indicate the magnitude of HIV replication and its associated rate of CD4+ T-cell destruction, while CD4+ T-cell counts indicate the extent of HIV-induced immune damage already suffered. Regular, periodic measurement of plasma HIV RNA levels and CD4+ T-cell counts is necessary to determine the risk of disease progression in an HIV-infected individual and to determine when to initiate or modify antiretroviral treatment regimens.
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