Since viral infections have been shown to influence T cell function by inducing hyperactivation and anergy [69], the frequency of CD8+ T cells as well as the expression of the T cell immunoreceptor with Ig and ITIM domains (TIGIT), TIM-3, programmed cell death protein 1 (PD-1), and CD69 was evaluated as surrogate markers by MSI. This evidence concerns the gene PDCD1 and viral infectious disease.