If an infant has open-mouth breathing, the following causes may be considered: 1) Nasal congestion in children, which can be caused by upper respiratory tract infections or rhinitis, which can lead to severe nasal congestion. 2) Adenoid hypertrophy in infants, which can also lead to open-mouth breathing. 3. Symptoms caused by pneumonia in infants, which can cause a significant increase in the number of breaths and symptoms such as the trigeminal sign, which can also cause open-mouth breathing due to hypoxemia in the infant.