Runny nose is a common phenomenon in 5-month-old babies, because the immunity and resistance of the baby’s body is relatively low at this time, and it is easy to be infected by various pathogens, thus causing a cold or influenza. The first symptom of a cold or flu is a runny nose, which is usually the initial symptom of a common cold if it is a simple clear nose. If the runny nose is yellow and accompanied by fever and other symptoms, it is usually caused by influenza virus. If your baby only has a little runny nose, it may be caused by a cold, so you can give your baby more warm water. Since a 5-month-old baby’s digestive system is not fully developed, try not to give him medicine if you can do so. However, if there is a yellow purulent nose, you should be alert to the phenomenon of influenza, and it is best to take your baby to the hospital in time for treatment. On the other hand, the phenomenon of yellow runny nose may also be caused by fire in the baby. If you are breastfeeding, you must pay attention to your diet, not to eat spicy and stimulating food, and try to eat lightly. You should pay attention to help your baby keep warm, add clothes at the right time, and keep the room air flowing.