AIDS is the stage of infection that occurs when your immune system is badly damaged and you become vulnerable to opportunistic infections. When the number of your CD4 cells falls below 200 cells per cubic millimeter of blood (200 cells/mm3), you are considered to have progressed to AIDS. (The CD4 count of an uninfected adult/adolescent who is generally in good health ranges from 500 cells/mm3 to 1,600 cells/mm3.) You can also be diagnosed with AIDS if you develop one or more opportunistic infections, regardless of your CD4 count.
Without treatment, people who are diagnosed with AIDS typically survive about 3 years. Once someone has a dangerous opportunistic illness, life expectancy without treatment falls to about 1 year. People with AIDS need medical treatment to prevent death.