The yellow phlegm of a cold may be caused by the following factors: 1. The yellow phlegm of a cold is caused by the infection of the respiratory tract with bacteria and other pathogens, and the infection of the trachea and bronchi with bacteria will lead to an increase in secretions in the trachea, resulting in the phenomenon of coughing up yellow phlegm. 2. It is also possible that the coughing up of yellow phlegm is caused by wind-cold and heat, accompanied by sneezing and runny nose, etc. 4. When the patient has the above symptoms, if the symptoms are mild, it can be relieved by drinking more water, if the symptoms are more serious, it is necessary to give anti-infection, cough and phlegm treatment.