Can I really tell how sick I am from the color of my mucus?
Sort of. The hue of your snot is more of a clue as to how hard your immune system is working to fend off an infection, whether bacterial or viral. Normally clear, mucus is made up of mostly water, as well as proteins, antibodies, and dissolved salts. Its purpose is to keep your nose from getting too dry and to protect your lungs from dust, allergens, bacteria, viruses, and other intruders. Even when we're at our healthiest, we have mucus in our nasal passages, but we typically don't notice and usually just swallow it.
Mucus that appears thicker and cloudier could be a sign of allergies. When you're sick, on the other hand, your mucus generally starts out clear and stringy and changes color as the infection progresses. It may first begin to turn heavy and white as the tissues in the nose become swollen and inflamed, impeding the flow of mucus and causing it to get drier. Thicker snot that's yellow or green indicates that your immune system has really kicked into fighting mode and has recruited more white blood cells, which contain enzymes that alter the color.
If you've been stuffed up with yellow or green mucus for a few days, you probably just have a cold (which is a viral infection), and it should clear up in 10 to 14 days. Drink fluids and take an over-the-counter decongestant while you wait it out. If your cold symptoms persist for longer than two weeks, or if you've developed additional symptoms, like fever or nausea, you may have bacterial sinusitis or pneumonia; your doctor can prescribe you antibiotics.
Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.