CTLA4 and viral infectious disease: It is well documented that molecules associated with T cell exhaustion, including T cell immunoglobulin and mucin domain 3 (TIM-3), lymphocyte-activation gene 3 (LAG-3), and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), are increased in some viral infections, such as HIV and hepatitis C, both during the acute and chronic phases [6,7].