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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.
Green or yellow phlegm is commonly caused by: Bronchitis: This usually starts off with a dry cough and eventually some clear or white phlegm. Over time, you may start coughing up yellow and green phlegm. This is a sign that the illness may be progressing from viral to bacterial. Coughing can last up to 90 days. Pneumonia: This is typically a complication of another respiratory issue. With pneumonia, you may cough up phlegm that is yellow, green, or sometimes bloody.