The goal of this prospective clinical study is to evaluate the safety and efficacy of Multi-Virus Specific T cells (LB-DTK-MV) in pediatric patients with systemic viral infection, including CMV, EBV, and BKV, after allogeneic hematopoietic stem cell transplantation (allo-HSCT). The main questions it aims to answer are:
* What is the maximum tolerated dose of LB-DTK-MV based on dose-limiting toxicity?
* What treatment emergent adverse events occur within 14 days after the second infusion?
* Is there a clinically significant reduction in CMV, EBV, and BKV viral loads within 14 days following the second infusion?
* Is there a clinically significant improvement in clinical symptoms within 14 days following the second infusion?
Participants will:
* Receive a single intravenous infusion of LB-DTK-MV during the baseline visit (low dose: 1x10\^7/m\^2; high dose: 2x10\^7/m\^2).
* Receive the second infusion of LB-DTK-MV intravenously at the same dose 14 days after the first infusion.
* Attend weekly follow-up visits at the clinic for 6 months after the first infusion.