CD4 and Opportunistic infection: Eventually CD4+ T cells decline to levels below 200 cells/μl in blood and infected individuals become susceptible to opportunistic infections such as Pneumocystis jiroveci (earlier named P. carinii), cytomegalovirus, Cryptosporidium spp., and Mycobacterium tuberculosis, and cancers such as Kaposi’s sarcoma [reviewed in Ref.