This study is a multicenter cross-sectional observational analysis of unused lower respiratory tract secretions collected from children with a history of HCT who are undergoing evaluation for pulmonary complications. Patients will be screened and enrolled by study coordinators at each site, who will collect and submit biospecimens as well as patient characteristics and clinical outcomes. Metagenomic NGS will be performed on these biospecimens in an attempt to identify microbial pathogens and markers of host response to infection, including immune activation, inflammation, and cell damage. This study will then correlate these results with patient characteristics, clinical microbiology test results, and clinical outcomes in order to evaluate the utility of metagenomics NGS in improving the diagnosis of LRTI in our pediatric HCT population.