title: To understand virus infection in children with HSCT and the common characteristics of virus infection at different stages of HSCT.
design: From June 2017 to March 2023, mNGS in the blood of children who underwent hematopoietic stem cell transplantation at National Children's Medical Center (Beijing Children's Hospital) was detected when they developed fever and the cause of fever was unknown, and the virus detection results of which were collected and summarized.