Cross-sectional studies of long COVID patients who had mild infection have found decreases in central memory [29] and effector memory CD4+ T cells [39], plus higher frequencies of exhausted CD4+ and CD8+ T cells [29], as well as increased expression of PD-1 on central memory T cells [39], an inhibitory molecule associated with T cell exhaustion in chronic viral infection and cancer. The gene discussed is CD4; the disease is infection.