Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

The human immunodeficiency virus (HIV) weakens the body's immune defenses by destroying CD4 (T-cell) lymphocytes, a type of white blood cell. T-cells normally help guard the body against attacks by bacteria, viruses and other germs.

When HIV destroys CD4 cells, the body becomes vulnerable to many different types of infections. These infections are called "opportunistic" because usually they only have the opportunity to invade the body when the immune defenses are weak. HIV infection also increases the risk of certain cancers, illnesses of the brain and nerves, body wasting, and death.

The range of symptoms and illnesses that can happen when HIV infection severely weakens the body's immune defenses is called acquired immunodeficiency syndrome or AIDS.

Since 1981, when doctors first recognized HIV/AIDS as a new illness, scientists have learned much about how a person becomes infected with HIV. The virus is spread through contact with an infected person's body fluids, especially through blood, semen and vaginal fluids. HIV can be transmitted:

  • During sex (anal, vaginal and oral)

  • By contaminated blood (by sharing or accidentally being stuck with a contaminated needle

  • Through transfusions before blood products started being screened for HIV in 1985)

  • By being born to a mother who is infected with HIV

Once inside the body, HIV particles invade CD4 cells and use the cells' own building machinery and materials to produce billions of new HIV particles. These new particles cause the infected CD4 cells to burst (lyse). The new particles can then enter the bloodstream and infect other cells.

Once someone is infected with HIV, the number of their CD4 cells continues to decrease. HIV is actively copying itself and killing CD4 cells from the time the infection starts. Eventually, the number of CD4 cells drops below the threshold level needed to defend the body against infections, and the person develops AIDS.

According to the World Health Organization (WHO), 35 million people worldwide are living with HIV. Survival has improved dramatically in developed countries. But that is not the case in many under developed countries.

More than 1,100,000 people living with HIV in the United States. While African Americans are 12% of the population, nearly 50% of those with HIV in the U.S. are African American. African American men are six times more likely to be infected with HIV than white men and African American women are 18 times more likely to be infected with HIV than white women.

In the U.S. today, about 25% of HIV infections are in women. Most of them were infected through sex with an infected man.

The CDC estimates that about 20% of people in the U.S. who have HIV don't know they are infected. It is important for people infected with HIV to know their status so they can get medical treatment before AIDS develops and they can take steps to prevent passing the virus to someone else.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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