Tom is 7 months old and is brought in to the GP surgery by his concerned father. He is worried that he has a fever of 38, has been wheezy and had a cough over the last few days. He is usually a happy child, is up to date with immunisations and has no other medical problems. He thinks it is a dry cough and admits he has had a slight runny nose. He is maintaining most of his usual oral intake. On examination, his respiratory rate is 40 but other observations are normal and diffuse bilateral wheeze and crackles are noted.
What is the most likely diagnosis?