A 4-month-old boy is brought to the emergency department in December. He has had a runny nose and a mild fever for 3 days. Over the last day he has developed a persistent cough and noisy breathing, and he is taking about half of his usual milk feeds. He was born at term and has no previous wheeze. His temperature is 37.9°C, respiratory rate 58/min and oxygen saturation 95% on air. There is mild subcostal recession, and auscultation reveals widespread fine inspiratory crackles with a high-pitched expiratory wheeze.

What is the most likely diagnosis?