CD8A and viral infectious disease: Exhausted T cells are perceived as T cells in unresponsive and nonfunctional states during chronic virus infection because the term ‘exhaustion’ was first used to describe the reduced capacity of effector CD8+ T cells to secrete inflammatory cytokines, such as IFNγ and TNF, as well as cytotoxic effector molecules, such as granzyme and perforin, all of which are hallmarks of cytotoxic effector T cells17.