Wheezing in children under 5 years of age is classified into 3 clinical phenotypes: (1) Early transient wheezing: Mostly seen in premature births and parental smokers, the wheezing is mainly due to environmental factors causing delayed lung development, which gradually matures with age, and most children gradually disappear within 3 years of age. (2) Persistent wheezing with early onset (defined as onset before 3 years of age): Children mainly present with recurrent wheezing associated with acute respiratory viral infections, with no personal atopic manifestations and no family history of allergic diseases. Wheezing symptoms usually persist until school age, and some patients remain symptomatic at 12 years of age. In children younger than 2 years of age, the cause of wheezing episodes is usually associated with infections such as respiratory syncytial virus, and in children older than 2 years of age, it is often associated with other viral infections such as rhinovirus. (3) Late-onset wheezing/asthma: These children have a typical atopic background, often with eczema, and asthma symptoms often extend and persist into adulthood, with typical asthma pathology in the airways.