Inclusion Criteria:  Parent/Guardian able to provide written informed consent Within 6 weeks of discharge from hospital for bronchiolitis Child aged 3-12 months at the time of consent to study A diagnosis of Bronchiolitis requiring a hospital admission (defined as more than 4 hours in hospital) Contactable for regular follow up by the research team  Exclusion Criteria:  Any previous hospital attendance for bronchiolitis More than one episode of healthcare professional-diagnosed wheeze prior to index bronchiolitis episode Premature gestational age less than 37 weeks Any severe chronic condition such as cystic fibrosis, sickle cell disease, severe developmental delay, immunodeficiency, or anything that has a significant impact on the respiratory tract (such as need for non-invasive ventilation) or increases vulnerability to respiratory tract infections. History of clinically significant neonatal disease (e.g. neonatal pneumonia, congenital lung abnormality, neonatal chronic lung disease) Genetic conditions that affect the immune system (e.g. Down's syndrome/Trisomy 21) Current regular oral montelukast or inhaled corticosteroid therapy or inhaled salbutamol therapy Current regular treatment with immunomodulatory drugs (e.g oral steroids) Known allergy or previous intolerance to study medication. Currently enrolled to another RCT. (Unless prior approval is given by PI) Sibling of a BLIPA participant (of the same household or family)