A Patient Is Diagnosed with AIDS When CD4 T-Cell Counts $\mu$

A patient is diagnosed with AIDS when CD4 T-cell counts _____.

A) rise markedly after T-cell activation
B) fall below the CD8 T-cell count
C) fall below 1000 cells/ $\mu$ l
D) fall below 500 cells/ $\mu$ l
E) fall below 200 cells/ $\mu$ l.