All HSCT patients at the St. Anna Children's Hospital undergo weekly viral quantitative PCR-screening for HAdV and CMV and weekly PB FACS (Fluorescence Activated Cell Sorter)-Analysis according to the local HSCT-diagnostic SOP (Standard Operating Procedure) from day -7 until day +100 Patients with HAdV or CMV viremia will receive preemptive treatment with either gancyclovir (in case of isolated CMV-viremia) or Cidofovir (in case of HAdV viremia or combined HAdV/CMV infection). In case of increasing viremia ≥ 1log despite antiviral treatment for two weeks or stable with 10E6 viral load and the absence of virus specific T-cells in the recipient, the treating physician will check, if the patient is eligible for seVirus-T-cell infusion (see inclusion criteria).