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Updated May 20, 2014

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HIV virions budding from a CD4+ T-Cell

Credit: National Institute of Allergies and Infectious Diseases (NIAID)

Question: What is HIV/AIDS?

Answer: HIV is the acronym for the human immunodeficiency virus. HIV is classified as a retrovirus, which is a type of virus that replicates within a host cell.

What is a Retrovirus?

A retrovirus is "retro" because it transcribes its genetic code in reverse. In most living organisms, including viruses, the genetic material is encoded from DNA to RNA. A retrovirus is unique in that it functions in the opposite direction, using its RNA coding to produce DNA within the host cell.

The newly produced DNA is then incorporated into the host's genetic coding, which it treats as its own, producing new retroviruses that go on to infect and kill other host cells.

In the case of HIV, the hosts are the so-called helper T-cells that comprise part of the body's immune system. Chief among these are CD4 cells, which effectively "signal" the body's immune defenses. By gradually depleting these cells, HIV weakens the body's immune response, leaving it vulnerable to infections it could otherwise destroy.

What Happens If You Are Infected?

HIV is primarily spread through sexual contact, transmission from mother to child, shared hypodermic needles, and contact with infected blood. HIV cannot be transmitted through sweat, tears or saliva.

Upon initial infection, HIV replicates vigorously, infecting and destroying a substantial number of CD4 cells. In response, the body's natural immune defenses are activated. The primary infection is gradually brought under control, while CD4 levels are restored (albeit to generally lower levels). However, HIV is not eliminated.

Instead, it goes into a period of latency, which can last anywhere from 8-12 years. During this time, the infected person may feel well and be asymptomatic, but the virus continues to replicate and deplete the body's immune function until it is eventually "compromised," leaving the body open to potentially life-threatening illnesses.

What is AIDS?

AIDS is the acronym for acquired immune deficiency syndrome. AIDS is not a disease, but rather the stage of HIV infection where the body's immune system is severely compromised, allowing for a variety of opportunistic infections to take hold. (Infections are considered "opportunistic" if they cause disease when immunity is impaired.)

Technically, AIDS is defined by either a CD4 count of under 200 cells per microliter (µL), or by the diagnosis of an AIDS-defining illness (a disease associated specifically with HIV infection). Normal CD4 counts range from between 500 to 1600 cells per µL.

If left untreated, the average survival time for a person with an AIDS diagnosis is 6-19 months. By contrast, a 35-year-old started on antiretroviral therapy (with a CD4 count of 350 or over) can expect a life span of 80 years or more, according to researchers at the UK Collaborative HIV Cohort Study.

Since it was identified in 1981, HIV has been attributed to the deaths of over 30 million people worldwide. Globally, there are more than 34 million people living with HIV today, 69% of which are in Sub-Saharan Africa. In the U.S., approximately 1.2 million people are infected, with an estimated 20% unaware of their status.


Biggar, R. "AIDS incubation in 1891 HIV seroconverters from different exposure groups." AIDS, November 1990; 4(11):1059-1066.

Zwahen, M. and Egger, M. "Progression and mortality of untreated HIV-positive individuals living in resource-limited settings." UNAIDS Obligation HQ/05/422204, June 2006.

May, M.; Gompels, M.; and Sabin, C. "Life expectancy of HIV-1-positive individuals approaches normal conditional on response to antiretroviral therapy: UK Collaborative HIV Cohort Study." Journal of the International AIDS Society. November 11, 2012; 15(4): 18078.

Joint United Nations Programme on HIV/AIDS. "UNAIDS Report on the Global AIDS Epidemic 2010." May 26, 2011. ISBN 978-92-9173-871-7; ref. UNAIDS/C1958E.

Joint United Nations Programme on HIV/AIDS. "UNAIDS Report on the Global AIDS Epidemic 2012." ISBN 978-92-9173-996-7; ref. UNAIDS/JC2417E.

Centers for Disease Control and Prevention. "HIV surveillance-United States, 1981-2008." Morbidity and Mortality Weekly Report. June 3, 2011; 60(21):689-693.


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