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Yellow phlegm in particular can scream infection. But, interestingly enough, the color does not come from infection. A yellow hue to the mucus is caused from white blood cells, according to Health.com. So, yellow mucus therefore isnât a result of an infection, but a result of the body fighting an infection. Coughing up yellow mucus therefore can be associated with battling the illness as opposed to being a symptom.
Home Â» Pulmonology Â» Phlegm Â». Coughing up green phlegm is a sure sign of a bacterial infection in the bronchi or lungs, but it could also indicate post nasal drip caused by a sinus infection. In case of a viral infection or allergic irritation of upper respiratory organs, you are more likely to experience clear mucus.