The target of the HIV-1 virus is a white blood cell called a CD4 cell. These blood cells are important because they tell other infection-fighting cells when to start working. HIV-1 infection lowers the number of CD4 cells (the CD4 count). When the number of CD4 cells drops to a certain level, the body’s immune system weakens. When the CD4 count drops below 200, or when opportunistic infections or cancers occur, a person with HIV-1 infection is said to have AIDS (Acquired ImmunoDeficiency Syndrome).

To assess the status of an individual’s HIV-1 infection, two laboratory markers are essential: CD4 count and HIV-1 viral load. This section of the site will explain in detail the role of HIV-1 viral load.