Phlegm is the thick, sticky mucus your body produces in your throat and lungs, especially when you're sick with a cold or the flu. When you feel that annoying need to clear your throat or cough something up, that's phlegm.

Your body makes phlegm all the time as part of keeping your airways healthy, but you usually don't notice it. When you catch a cold, though, your body goes into overdrive and produces extra phlegm to trap germs and help flush them out. That's why you might cough up phlegm when you're sick, or feel it collecting in the back of your throat.

Phlegm can be clear, white, yellow, or even greenish, depending on what's going on in your body. Doctors sometimes ask about the color because it gives them clues about whether you have an infection. While it's definitely gross to think about, phlegm is actually your body's clever way of protecting itself and fighting illness.