Acquired immunodeficiency syndrome (AIDS) is the most severe stage of HIV infection. HIV attacks and destroys a type of white blood cell called CD4+ cells, or helper cells. These cells are an important part of the immune system. If HIV is not treated, it will usually get worse over time and cause AIDS.

A person who has HIV is diagnosed with AIDS when they have one or both of the following:

• A CD4+ cell count below 200 cells per microlitre (mcL) of blood.
• Certain infections or cancers that are usually seen only in people who have problems with their immune system. Examples include infections such as Pneumocystis pneumonia and cancers such as Kaposi sarcoma.

Treating HIV can help the immune system stay healthy. This may delay or prevent AIDS.