In an HIV-free person, the normal range for a CD4 cell count is 500 - 1500 cells/ml of blood. When that same individual first becomes HIV positive, their CD4 count drops, sometimes drastically. 

This drastic drop in CD4 count is caused because the HIV attacks and destroys the host CD4 as it progresses, while also hampering the production of new CD4 cells.   With treatment, the virus can be suppressed, which stabilizes the CD4 count. However, without treatment, the CD4 count can keep dropping over the subsequent years.