Level 1: Causes and treatment methods of baby\’s runny nose 1. Causes of baby\’s runny nose There may be many reasons for baby\’s runny nose, the most common ones are colds, allergies, climate change, etc. When a baby is infected with a virus or bacteria, the body produces too much mucus, which can lead to a stuffy and runny nose. When there are too many dust mites, pollen and other substances in the air, it can cause allergic rhinitis in children.