Phlegm is a thick, sticky substance produced by the mucous membranes in the respiratory system. It helps trap dust, bacteria, and other particles, preventing them from entering the lungs. Phlegm is often associated with illnesses like the common cold or bronchitis, where the body produces more mucus to fight off infections. When a person is healthy, phlegm is usually clear or white. However, it can change color to yellow or green when an infection is present. Excessive phlegm can lead to coughing and discomfort, prompting individuals to seek relief through cough syrups or other remedies.