My son (14yrs) has started having cold and cough from last 2-3 months, he have cold (running nose for 3-4 days) and after that over/gets ok, then start coughing and which last for week, after that too in a day few times have cough. After 15-20 days again this start. Had done chest x-ray also done, and all ok in that. Please let know why this happening frequently and what medicine can be taken to avoid not to have this in regular intervals.