The human immunodeficiency viruses (HIV) are two species of Lentivirus (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS),[1][2] a condition in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive.[3] Without treatment, average survival time after infection with HIV is estimated to be 9 to 11 years, depending on the HIV subtype.[4] In most cases, HIV is a sexually transmitted infection and occurs by contact with or transfer of blood, pre-ejaculate, semen, and vaginal fluids. Research has shown (for both same-sex and opposite-sex couples) that HIV is untransmittable through condomless sexual intercourse if the HIV-positive partner has a consistently undetectable viral load.[5][6] Non-sexual transmission can occur from an infected mother to her infant during pregnancy, during childbirth by exposure to her blood or vaginal fluid, and through breast milk.[7][8][9][10] Within these bodily fluids, HIV is present as both free virus particles and virus within infected immune cells.
HIV infects vital cells in the human immune system, such as helper T cells (specifically CD4+ T cells), macrophages, and dendritic cells.[11] HIV infection leads to low levels of CD4+ T cells through a number of mechanisms, including pyroptosis of abortively infected T cells,[12] apoptosis of uninfected bystander cells,[13] direct viral killing of infected cells, and killing of infected CD4+ T cells by CD8+ cytotoxic lymphocytes that recognize infected cells.[14] When CD4+ T cell numbers decline below a critical level, cell-mediated immunity is lost, and the body becomes progressively more susceptible to opportunistic infections, leading to the development of AIDS.
https://en.wikipedia.org/wiki/HIV
Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV),[9][10][11] a retrovirus.[12]
The World Health Organization first proposed a definition for AIDS in 1986.[27] Since then, the WHO classification has been updated and expanded several times, with the most recent version being published in 2007.[27] The WHO system uses the following categories:
- Primary HIV infection: May be either asymptomatic or associated with acute retroviral syndrome[27]
- Stage I: HIV infection is asymptomatic with a CD4+ T cell count (also known as CD4 count) greater than 500 per microlitre (µl or cubic mm) of blood.[27] May include generalized lymph node enlargement.[27]
- Stage II: Mild symptoms, which may include minor mucocutaneous manifestations and recurrent upper respiratory tract infections. A CD4 count of less than 500/µl[27]
- Stage III: Advanced symptoms, which may include unexplained chronic diarrhea for longer than a month, severe bacterial infections including tuberculosis of the lung, and a CD4 count of less than 350/µl[27]
- Stage IV or AIDS: severe symptoms, which include toxoplasmosis of the brain, candidiasis of the esophagus, trachea, bronchi, or lungs, and Kaposi's sarcoma. A CD4 count of less than 200/µl[27]
The United States Center for Disease Control and Prevention also created a classification system for HIV, and updated it in 2008 and 2014.[114][115] This system classifies HIV infections based on CD4 count and clinical symptoms, and describes the infection in five groups.[115] In those greater than six years of age it is:[115]
- Stage 0: the time between a negative or indeterminate HIV test followed less than 180 days by a positive test.
- Stage 1: CD4 count ≥ 500 cells/µl and no AIDS-defining conditions.
- Stage 2: CD4 count 200 to 500 cells/µl and no AIDS-defining conditions.
- Stage 3: CD4 count ≤ 200 cells/µl or AIDS-defining conditions.
- Unknown: if insufficient information is available to make any of the above classifications.
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