A Study of Rilematovir in Infants and Children and Subsequently in Neonates Hospitalized With Acute Respiratory Tract Infection Due to Respiratory Syncytial Virus (RSV)

Inclusion criteria: * The participant weighs within greater than or equal to (\>=) 2.4 kilograms (kg) and less than or equal to (\<=) 24.6 kg * Each participant's parent(s) (preferably both if available or as per local requirements) or their legally acceptable representative(s) has/have signed an informed consent form (ICF) indicating that (s)he understands the purpose of, and procedures required for, the study; is willing for their child to participate in the study; with regards to the concomitant medication, the lifestyle consideration and study procedures and assessments to be performed by the parent(s)/caregiver(s) as well as those by the investigator/study site personnel * The participant has an acute respiratory illness with at least 1 of the signs/symptoms within 24 hours prior to start of screening and at screening, as evaluated by the investigator in Upper respiratory tract infection: nasal congestion or rhinorrhea; and Lower respiratory tract infection: increased respiratory effort (as evidenced by subcostal, intercostal or tracheosternal retractions, grunting, head bobbing, nasal flaring, or tachypnea), wheezing, cough, cyanosis, or apnea; and systemic/general: feeding difficulties (defined as \<75 percent \[%\] intake of normal food amounts); dehydration; fever; disturbed sleep, or disturbed activity level (irritable/restless/agitated/less responsive). Cough or wheezing cannot be the only LRTI sign/symptom present, that is, at least one other LRTI sign/symptom ne…