Phlegm. It's that nasty, gross stuff you get in your mouth when you cough. Phlegm is mucus that's produced in your chest, and when you get sick, it can be different colors. You usually don't have much phlegm, called sputum, when you cough it out come up from your chest except when you're sick. Colors of your sputum can mean different things. Even if you have a particular color of sputum, you may not be able to tell precisely what's causing it -- not without your doctor's diagnosis, which may require tests.
Yellow phlegm typically means you have an infection of some variety. The infection may be either bacterial or viral. The yellow color is caused by white blood cells fighting the infection. If the infection continues, it changes to green. Yellow phlegm is usually a sign of one of the following:
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