Understanding the Difference Between a Virus and the Stage of Disease

Electron micrograph of HIV-1 (in yellow) budding from an infected CD4+ T-cell. Credit: National Institute of Allergies and Infectious Diseases (NIAID)

HIV is the acronym for the human immunodeficiency virus. HIV is classified as a retrovirus, a type of virus which replicates within a host cell by hijacking its genetic machinery in order to make multiple copies of itself.

AIDS is the acronym for acquired immunodeficiency syndrome and is the stage of HIV infection where a person's immune system is fully compromised, leaving the body open to a wide range of potentially deadly diseases.

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 onto infect and kill other host cells.

The primary target for HIV infection are white blood cells called "helper" T-cells. Chief among these are CD4+ T-cells, whose job it is to trigger the body's immune response. By gradually depleting these cells, HIV weakens the immune system to such a degree that it is no longer able to fight off infections otherwise harmless to healthy individuals.

What Happens If You Are Infected With HIV?

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+ T-cells. In response, the body's natural immune defenses are activated. While the primary infection is gradually brought under control and the CD4 levels are restored (albeit to generally lower levels), the virus itself is not eliminated.

Instead, it goes into a period of latency, which can last anywhere from eight to 12 years. During this time, the infected person may feel well and be asymptomatic, but the virus will continue to replicate, while hiding itself within hidden cells and tissues called latent reservoirs.

What Happens When a Person Is Diagnosed With AIDS?

AIDS is not a disease per se 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. 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 between six and 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 research from 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 35 million people living with HIV today, 69% of whom are in Sub-Saharan Africa.

In the U.S., approximately 1.2 million people are infected with HIV, according to surveillance from the Centers for Disease Control and Prevention (CDC) in Atlanta. Of these, 20-25% are estimated to be undiagnosed.

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

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