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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.
Some researchers have called into question the practice of prescribing antibiotics based on phlegm color. A recent study found that people with green or yellow phlegm who received antibiotics didn't recover any faster from their sickness than people without mucous and who didn't get the medications.