Specifically, there are three criteria for HIV diagnosis (including AIDS): (1) immunological criteria used for a child younger than 5 with confirmed HIV infection: for children < 12 months, %CD4+ <30; for those aged 12–35 months, %CD4+ <25; and for those aged 36–59 months, %CD4+ <20; (2) clinical criteria used for children and adults with confirmed HIV infection: definitive or presumptive diagnosis of any stage 3 or stage 4 condition; and (3) immunological criteria used for children and adults 5 years or older with confirmed HIV infection: a CD4 count less than 350 per mm3 of blood. Here, CD4 is linked to HIV infectious disease.