There are many reasons that cause the baby from clear nasal discharge to yellow nasal discharge, which may be caused by acute upper respiratory tract infection combined with bacterial infection or rhinitis. 1. Acute upper respiratory tract infection: acute upper respiratory tract infection is caused by viral infections, the initial manifestation of the runny nose, if the combination of bacterial infections, there may be the phenomenon of yellow nasal discharge, often accompanied by nasal congestion, sore throat, cough and other symptoms. 2. Rhinitis: bacterial infection caused by rhinitis, may also lead to the baby from the phenomenon of runny nose to runny yellow nose, can be accompanied by nasal congestion, sneezing and other symptoms. The baby from clear nasal discharge to yellow nasal discharge may also have other reasons, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, to give targeted treatment or treatment.