Autologous chimeric antigen receptor (CAR)-T cell therapy has revolutionized cancer treatment and allogeneic CAR-T cell therapy is poised to advance this revolution. CAR-T cell therapy faces some concerns regarding adventitious agents, which can threaten the safety of patients. Human herpesviruses 6 and 7 (HHV-6 and HHV-7) have become increasingly notable in this context, as they carry a risk with severe health consequences. This review explores these virus reactivations in CAR-T cell therapy and discusses mitigation strategies during allogeneic CAR-T cell manufacturing. We provide an overview of prevention and testing strategies, genetic engineering applications, and chemical substances with potential for interventions. This review aims to enhance understanding of HHV reactivation and improve the safety of allogeneic CAR-T cell therapies.