Final answer:
Opportunistic infections become a significant risk when the CD4 T cell count in HIV-infected individuals drops below 200 cells/μL, which is the level at which AIDS is diagnosed.
Step-by-step explanation:
The level of CD4 T cells serves as a critical indicator of immune system health in individuals infected with HIV. Opportunistic infections tend to take hold when the CD4 T cell count falls below a certain threshold. Specifically, patients are diagnosed with AIDS, and are at high risk for opportunistic infections, when their CD4 T-cell count drops below 200 cells/μL. At this point, the individual has a severely compromised immune system and is susceptible to a range of infections and diseases that are typically uncommon in those with a functioning immune system.
A healthy individual has a CD4+ T cell count of about 1,000 cells per microliter. Declines in CD4+ T cells below 400 cells per microliter may not yet be considered AIDS, but they can still cause the person to be immunodeficient and therefore more prone to certain infections. Continuous monitoring of the CD4+ T cell count through flow cytometry is vital for adjusting HIV treatments to prevent the fall to critically low levels.