Viral upper respiratory infection
Respiratory syncytial virus (RSV)
Bronchiolitis
Chlamydia trachomatis Pneumonia
Reactive airway disease (Asthma-like symptoms-wheezing)
Croup (Parainfluenza)
Pneumonia
Pertussis
Sepsis
Metapneumovirus
Influenza
Pneumothorax
Foreign body aspiration (esp. after 6 mo of age)
Congenital heart disease
Further Reading
https://www.ncbi.nlm.nih.gov/books/NBK154448/
https://www.aafp.org/afp/2004/0201/p535.html