Green, yellow, brown... As you've probably seen from peering into your soggy tissues (we know you do too!), these are the many colours of mucus.
Besides being grossly fascinating, your phlegm might offer clues to what's going on in your airways.
According to Dr Steve Yang, a specialist in respiratory medicine and consultant from Raffles Internal Medicine Centre, phlegm (or sputum) and mucus colours can say something about your illness.
If your mucus is... green or yellow
When you have a viral or bacterial infection, your immune system detects the foreign invaders and sends out neutrophils (white blood cells) to fend them off.
These cells contain a protein that gives your phlegm its green colour.
Besides flu, conditions such as bronchitis and pneumonia can cause sputum to look like this too.
If your phlegm is... red or brown
Ouch! Your throat must be inflamed and bleeding a little.
Reddish sputum suggests a more recent bleed than a brownish one.
You don't have to be too worried about this unless you're coughing blood - see a doctor within 24 hours.
An infection can trigger this. Less common causes include tubercolosis and lung cancer.
If your sputum is... clear
Good news: It's a part of your body's normal mucus production.
However, if you're coughing it out in excess, it's best to check for underlying conditions like asthma.
If you're having or just fought the flu, clear phlegm does not mean you're not contagious.
You can start infecting others a day before symptoms appear, and after an episode, you can still spread the bug for five to 10 days.
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