Usually, a runny nose is due to the patient’s symptoms of a cold or rhinitis. If the cause of the cold is a change from clear nasal mucus to yellow nasal mucus, it means that the symptoms of the cold are not getting better, but are aggravated, and a bacterial infection may have occurred. Some people will have a runny nose when they have a cold, the early stage of the cold is clear clear nasal mucus, which indicates that the cold evil only invaded the surface of the body, when the cold evil invasion of the body, the color of the nasal mucus will gradually aggravate from clear to yellow. 95% of the common cold is caused by viral infections, in the process of viral rhinitis, may be secondary to bacterial infections, the nasal mucus will be changed from clear nasal mucus to yellow nasal mucus. If the secondary bacterial infection will appear other comorbidities, such as may cause sinusitis, so the snot from clear to yellow is not a sign of improvement, often may be combined with bacterial infections, common bacteria including Staphylococcus aureus, hemolytic streptococcus. In addition, it can also be seen in the secondary infection of other pathogens, such as mycoplasma, chlamydia, etc. At this time, you can test the blood routine, if necessary, take the respiratory secretions to do pathogen culture, as well as drug sensitivity test, to clarify the possible existence of pathogens and choose sensitive anti-inflammatory drugs. Therefore, the nasal mucus from clear to yellow indicates that the symptoms are aggravating, there is an infection, need to be treated with antibiotics under the guidance of the doctor. At the same time, you should have enough rest, drink plenty of water, pay attention to warmth, avoid getting cold, to prevent the disease from recurring.