 HIV is characterized by persistent immune system activation, and early in the course of infection the body establishes an immune activation set point related to the amount of HIV in the blood stream. This set point affects the rate of CD4 cell loss. Without CD4 cells, or with very low levels of CD4 cells, the body cannot fight off illness. This is known as immunodeficiency. If left untreated HIV can lead to extreme immunodeficiency and AIDS. 