A 4-month-old infant presents with a 3-day history of coryza, low-grade fever, and a harsh cough, followed by increasing respiratory distress with audible wheeze and tachypnoea. On examination, respiratory rate is 65/min, there are subcostal and intercostal retractions, and widespread fine inspiratory crackles and expiratory wheezes on auscultation. Oxygen saturation is 90% on room air. What is the most likely causative organism?