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 upper respiratory tract infection (URTI), the effect of presatovir on development of lower respiratory tract complication, being free of any supplemental oxygen progression to respiratory failure, and pharmacokinetics (PK), safety, and tolerability of presatovir.