CD8A and viral infectious disease: Aging is a well-known important risk factor for increased severity of COVID-19 and is characterized by changes in adaptative immunity with a decreased T cell response to viral infection, a decreased generation of CD3+ and naïve T cells, inversion of blood CD4+:CD8+ T cell ratio through decreasing blood CD8+ T cells, and enhanced proliferation of regulatory T cells (Treg) [93] that are immune-suppressive cells.