Viral upper respiratory tract infection: This is the most common cause of congestion in infants, often caused by viruses such as rhinovirus, coronavirus, or respiratory syncytial virus (RSV). The symptoms typically include runny nose, cough, and sometimes fever.

Allergic rhinitis: Although less common in infants, allergic reactions to environmental allergens like dust mites, pet dander, or pollen can cause congestion.
Anatomical issues: Enlarged adenoids or a deviated septum can cause nasal congestion in infants, though these are less common causes.
Gastroesophageal reflux disease (GERD): Stomach acid reflux can irritate the nasal passages and cause congestion.

Bacterial sinusitis: Though less common than viral infections, bacterial sinusitis can lead to serious complications if not treated promptly. Symptoms may include high fever, nasal discharge, and facial pain (though facial pain may be difficult to assess in infants).
Choanal atresia: A congenital condition where the back of the nasal passage is blocked, which can cause significant respiratory distress in newborns and infants, especially if the atresia is bilateral.
Foreign body in the nose: Infants may insert small objects into their noses, leading to congestion and potential infection.

Cystic fibrosis: A genetic disorder that can cause respiratory and digestive problems, including nasal congestion due to thick, sticky mucus.
Immune deficiencies: Certain immune system disorders can make infants more susceptible to infections that cause congestion.
Tumors or cysts: Rarely, nasal congestion in infants can be caused by tumors or cysts in the nasal passages, which can obstruct breathing and cause other symptoms.