This study will evaluate the effect of presatovir on respiratory syncytial virus (RSV) viral load in autologous or allogeneic hematopoietic cell transplant (HCT) recipients with an acute RSV lower respiratory tract infection (LRTI), the effect of presatovir on being free of any supplemental oxygen, and rates of respiratory failure and all-cause mortality, and pharmacokinetics (PK), safety, and tolerability of presatovir.