Pathogenesis and Natural History

Acquired Immunodeficiency Syndrome (AIDS) was first recognized in the United States in 1981 and is caused by the retrovirus human immunodeficiency virus (HIV).  The structure of the virus consists of ribonucleic acid (RNA) and reverse transcriptase within the core capsid protein surrounded by an outer lipid membrane or viral envelope. 

After infection with the virus, the person may remain asymptomatic or develop acute retroviral syndrome within days to weeks.  This primary infection is characterized by a flu-like illness with fever, headache, fatigue, muscle aches, sore throat, rash, or generalized lymphadenopathy which may last from days to weeks.

In HIV-infected persons, the immune system recognizes the virus proteins as foreign material and produces antibodies through complex mechanisms involving the white blood cells and the virus. Usually within an average of 25 days to a maximum of three months from infection, the number of antibodies against the virus rises to detectable levels.  At this time seroconversion can be detected through HIV antibody tests.

After the primary infection, most patients remain asymptomatic for months to years.  Although this is called a period of clinical latency, there is continued replication of the virus and destruction of immune cells.

The CD4 positive T-helper cell is the primary target for HIV infection because the virus is attracted to the CD4+ surface marker. Because the CD4+ T cell coordinates a number of immunologic functions, a loss of these cells leads to a progressive impairment of the immune response. Persons who are infected with HIV, and who are diagnosed with life-threatening opportunistic diseases and cancers, or have CD4+ absolute cell counts <200 cells/mm3, or <14% of total lymphocytes meet criteria for CDC-defined AIDS.

Without therapy, the median time from infection with HIV to the development of AIDS-related symptoms has been approximately 10 to 12 years with wide variability.  Left untreated, a person with CD4 count less than 200 has a median survival of 3.7 years and after an AIDS-defining illness, the median survival is 1.3 years.

*Candida esophagitis, tracheitis, or bronchitis; extrapulmonary coccidioidomycosis, cryptococcosis, or histoplasmosis; diarrhea > 1month due to cryptosporidiosis or isosporosis; CMV other than liver, spleen, or LN; mucocutaneous HSV >1 month or HSV of esophagus, bronchus, lungs; disseminated Mycobacterium avium complex; tuberculosis; pneumocystis; recurrent bacterial pneumonia; recurrent salmonellosis; toxoplasmosis

+Invasive cervical cancer, dementia, wasting, Kaposi’s sarcoma, lymphoma (Burkitt’s, immunoblastic, primary CNS), progressive multifocal leukoencephalopathy.

Types of HIV

HIV-1 is the predominant type of HIV in the United States.  HIV-2 is also a retrovirus that causes a similar immunodeficiency syndrome but is seen primarily in patients in West Africa.  Despite HIV-2 being rarely reported in the US and mostly in patients who came from endemic areas or had exposure to individuals from endemic areas, all U.S. blood donations have been tested with a combination HIV-1/HIV-2 enzyme immunoassay test kit that is sensitive to antibodies of both viruses. According to some reports, HIV-2 has a longer incubation period than HIV-1. However, prevention aimed at HIV-1 can also help prevent and control the spread of HIV-2.

Modes of Transmission

HIV is transmitted most commonly through contact with infected blood such as with injection drug use or with semen or vaginal secretions during sexual intercourse (vaginal, oral or anal).  In addition, infected pregnant women can pass HIV to their baby during pregnancy or delivery, as well as through breast-feeding.

Common misperceptions about HIV have included the virus living in the environment or in the air, transmission through kissing, biting, contact with saliva, tears or sweat, and insects (such as a mosquito). Unless there has been a potential for contact with blood, semen, vaginal, other body fluids visibly contaminated with blood, or body fluids to which Universal Precautions apply, there have not been any cases reported where HIV was transmitted in any of these ways. HIV can only survive outside of the human body for a very short period of time.

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