Infections are a major cause of morbidity and mortality in patients undergoing hematopoietic stem cell transplant (HSCT). The purpose of this study is to evaluate if metagenomic next-generation sequencing (mNGS) can detect microbial signatures in people undergoing HSCT, and if microbial identification can be correlated with clinical features of infection (e.g., fever). Participants undergoing HSCT as part of other studies at the NIH Clinical Center (CC) will provide blood before the transplant and through 6 months after. Total nucleic acid will be extracted from plasma and subjected to mNGS. The primary objective of this study is to investigate if by using plasma and an mNGS approach, we can detect bacterial, fungal, protozoan, or viral DNA/RNA over time, in immunocompromised patients undergoing transplantation. Secondary objectives are to: (1) To correlate microbial identification with episodes of fever or clinical suspicion of infection; and to (2) correlate change in microbial signatures in patients with suspected immune reconstitution inflammatory syndrome. The study is conducted at the NIH Clinical Center. Participants, aged 3 years and older, on other research studies at the NIH CC who are undergoing HSCT are invited to take part of this study. Expected participation is up to six months.