Acquired Immune Deficiency Syndrome (AIDS)
 AIDS is the result of damage to the immune system after infection with HIV (Human Immunodeficiency Virus) and represents the most advanced stages of HIV. AIDS involves the diminished function of the immune system as critical immune cells (especially CD4 cells) are infected and destroyed. The immune system is no longer able to guard against illness, making the person vulnerable to other infections (including opportunistic infections) and cancers. People with HIV may experience different clinical problems, depending on which specific infections or cancers they develop.

Clinical definition: In the United States, the Centers for Disease Control and Prevention (CDC) defines AIDS as a CD4 cell count below 200 cells/mm3, or the presence of at least one opportunistic illness in an HIV-positive individual.
 Human Immunodeficiency Virus (HIV)
 HIV is the cause of AIDS. There are two major types, HIV-1 and HIV-2, and various subtypes of each major type. HIV-1 is prevalent throughout most of the world, while HIV-2 is found primarily in West Africa. HIV is a virus that replicates within a cell and copies its genetic material into the genetic material of human cells, thereby infecting cells for the rest of their lifespans.

Through mechanisms still not fully understood, HIV prevents the immune system from working properly. HIV infects CD4 cells, which coordinate the immune system's fight against infection. Many CD4 cells are actually destroyed by being infected. Other CD4 cells no longer work properly. As HIV continues to replicate, the HIV-infected person’s immune system eventually is unable to control the infection, leading to immune deficiency. However, a positive HIV test result does not mean a person has AIDS. A person is defined as having AIDS when his or her CD4 cell count is below 200 cells/mm3, or the presence of at least one opportunistic illness is diagnosed.

More information:
• Symptoms
• Diagnosis
• Treatment
• Transmission