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[Display omitted] • An update on lymphocytopenia induced by severe SARS-CoV disease and influenza A infections. • Apoptosis and sequestration are reportedly amongst two principal modes of CD4+/CD8+ leukodepletion. • A novel "Index Severity Score" of infection was classified in conjunction with depleted T -cell counts. • A total T -cell threshold of <=560 cell/uL, a predictor of advanced infection, escalated mortality; and a tool guiding clinical trials of new therapeutics. • Future research in progress to identify quantifiable relationships between T -cell counts, cytokines, inflammatory and coagulation markers in conjunction with disease severity. We proposed T -cell lymphocytopenia as a strategic predictor of serious coronavirus and influenza infections. Our preeminent goal was to determine whether a degree of T -cell lymphopenia would identify a distinct threshold cell count to differentiate between severe and non-severe infections. We codified an Index Severity Score to exploit an association between T -cell cytopenia and the grade of disease activity. A T -cell count of 560 cells/uL or below signified a trend towards advanced disease. (1.) The T -cell threshold > 560 cells/uL discriminated 85.7 % specificity of the lesser viral infections and <=560 cells/uL identified 100 % sensitivity of severe infections or death. (2.) The positive predictive value of this threshold test was 92.9 %. (3.) T -cell apoptosis and sequestration are two of the primary mechanisms of T -cell lymphodepletion. (4.) There is potential for the T -cell threshold at <=560 cells/uL to become a standard to differentiate disease severity. (5.) The T-cell threshold should be tested further against flow cytometry of CD4+, CD8+ counts of individual patients. (6.) Future research should explore correlations between the T -cell threshold, medical outcomes of treatment, Cytokine Release Syndromes, cytokine levels, inflammatory and coagulation markers. [ABSTRACT FROM AUTHOR]