HIV is one of the human retroviruses of the family Lentivirus; there are solid data that humans have acquired HIV from other primates via cross species transmission. After exposure to HIV, a symptomatic illness termed primary HIV infection (PHI) occurs in one to four weeks in more than half the patients; however, clinical immunodeficiency or AIDS develops after a long period of clinical latency that follows PHI. Clinical latency is a dynamic period where active viral replication occurs and immunologic abnormalities begin to emerge, most notable of which are quantitative and qualitative defects in the CD4+ T-lymphocyte pool. The average length of the latency period in adults is estimated to be about 10 years. Opportunistic infections (OIs) are the hallmarks of progression to AIDS and usually occur with CD4 counts of less than 200 cells/mm3.
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