A person is diagnosed with AIDS when they have developed an AIDS related condition or symptom, called an opportunistic infection, or an AIDS related cancer. The infections are called ‘opportunistic’ because they take advantage of the opportunity offered by a weakened immune system.
It is possible for someone to be diagnosed with AIDS even if they have not developed an opportunistic infection. AIDS can be diagnosed when the number of immune system cells (CD4 cells) in the blood of an HIV positive person drops below a certain level.