what is phlegm caused by
Phlegm symptoms are commonly caused by bacterial or viral infections of the lungs and upper airways. Healthy lungs are covered with a thin layer of mucus.When threatened by infectious germs or damaging foreign substances, such as smoke or dust, the lungs produce additional mucus.nfectious causes of phlegm symptoms. Phlegm symptoms can be caused by various viral and bacterial infections including: Acute bronchitis (inflammation of the bronchial tubes usually caused by a viral infection or sometimes a bacterial infection). Common cold (viral infection).