Facts about HIV/AIDS

What is HIV?

HIV is the human immunodeficiency virus that causes AIDS. A member of a group of viruses called retroviruses, HIV infects cells and uses the energy and nutrients provided by those cells to grow and reproduce

What is AIDS?

AIDS (acquired immunodeficiency syndrome) is a disease in which the body’s immune system breaks down and is unable to fight off certain infections, known as “opportunistic infections,” and other illnesses that take advantage of a weakened immune system.

When a person is infected with HIV, the virus enters the body and lives and multiplies primarily in the white blood cells. These are the immune cells that normally protect us from disease. The hallmark of HIV infection is the progressive loss of a specific type of immune cell called T-helper or CD4 cells.

As the virus grows, it damages or kills these and other cells, weakening the immune system and leaving the individual vulnerable to various opportunistic infections and other illnesses, ranging from pneumonia to cancer. The U.S. Centers for Disease Control and Prevention defines someone as having a clinical diagnosis of AIDS if they have tested positive for HIV and meet one or both of these conditions:

  • They have experienced one or more AIDS-related infections or illnesses;
  • The number of CD4 cells has reached or fallen below 200 per cubic millimeter of blood (a measurement known as T-cell count).

In healthy individuals, the CD4 count normally ranges from 450 to 1200.

In some people, the T-cell decline and opportunistic infections that signal AIDS develop soon after infection. In most people they remain asymptomatic for 10 to 12 years. As with most diseases, early medical care can help prolong a person’s life.


Certain body fluids from an HIV-infected person can transmit HIV.

These body fluids are:

  • Blood
  • Semen (cum)
  • Pre-seminal fluid (pre-cum)
  • Rectal fluids
  • Vaginal fluids
  • Breast milk

These body fluids must come into contact with a mucous membrane or damaged tissue or be directly injected into your bloodstream (by a needle or syringe) for transmission to possibly occur. Mucous membranes are the soft, moist areas just inside the openings to your body. They can be found inside the rectum, the vagina or the opening of the penis, and the mouth.

For more information, see CDC’s HIV Basics: HIV Transmission.


Antiretroviral Therapy (ART) uses different kinds of medication to keep HIV from growing and multiplying.  

For more information, see CDC's Overview of HIV Treatments.