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The '''Human Immunodeficiency Virus''' is a chronic disease that gradually weakens the immune system by disabling and killing "T-helper ", which normally signal other constituents of the immune system to perform their respective roles. When HIV reduces the number of T-helper cells below a certain threshold, an infected person is vulnerable to opportunistic infections and cancers that typify Acquired ImmunoDeficiency Syndrome, or AIDS. <ref>[http://www.niaid.nih.gov/factsheets/howhiv.htm]</ref> It is transmitted primarily through sexual contact with an infected person, sharing of needles or syringes, and, less frequently, through transfusion of infected blood or blood clotting factors. <ref>[http://www.cdc.gov/hiv/resources/factsheets/transmission.htm]</ref>.   

The '''Human Immunodeficiency Virus (HIV)''' is a chronic disease that gradually weakens the immune system by disabling and killing "T-helper ", which normally signal other constituents of the immune system to perform their respective roles. When HIV reduces the number of T-helper cells below a certain threshold, an infected person is vulnerable to opportunistic infections and cancers that typify Acquired ImmunoDeficiency Syndrome, or AIDS. <ref>[http://www.niaid.nih.gov/factsheets/howhiv.htm]</ref> It is transmitted primarily through sexual contact with an infected person, sharing of needles or syringes, and, less frequently, through transfusion of infected blood or blood clotting factors. <ref>[http://www.cdc.gov/hiv/resources/factsheets/transmission.htm]</ref>.   

HIV is a retrovirus, and integrates its genetic information into the host cell's genome.

HIV is a retrovirus, and integrates its genetic information into the host cell's genome.