Aids is the second stage of HIV infection. AIDS develops after HIV has severely weakened the immune system of the body. AIDS is diagnosed when a person with HIV develops 1+ opportunistic infections or when the number of CD4 cells in their blood falls below 200 cells per cubic millimeter of blood (200 cells/mm3), according to HIV.Gov. (CD4 counts range from 500 to 1,600 cells/mm3 in those with a healthy immune system.)”