Key Inclusion Criteria:  Children aged 6-24 months Good health based on history, physical examination, and medical record review, without evidence or suspicion of chronic disease Seronegative to RSV, as defined by serum nAb titer below the threshold described in the study protocol and operations manual Written informed consent provided by parent(s)/guardian(s)  Key Exclusion Criteria:  Known or suspected chronic illness, particularly cardiopulmonary (including asthma or reactive airways disease), genetic or metabolic, hepatic, renal, infectious (including recurrent or chronic sinusitis), or immunodeficiency Prior lab-confirmed RSV infection Household or close contact (including but not limited to daycare) during the 21 days post-inoculation with anyone < 6 months old or immunocompromised (applies to first study inoculation) Nasal obstruction (including due to anatomic/structural causes, acute or chronic rhinosinusitis, or other causes) Receipt of immunoglobulins, monoclonal antibodies and/or any blood products, or ribavirin within 6 months prior to study inoculation, or planned use during study period Receipt of an investigational RSV vaccine at any time Any other condition that, in the judgment of the investigator, would be a risk to subject's safety and/or may interfere with study procedures or interpretation of results