Acute bronchitis is characterized by the presence of phlegm and is caused by pathogen infections such as viruses, bacteria, mycoplasma, and chlamydia. These infections lead to congestion and edema in the airway mucosa. During this process, goblet cells proliferate, and mucus glands increase their secretion leading to an increase in phlegm. The excess phlegm can encapsulate pathogens and dust particles, which are then expelled from the body through the movement of cilia, a normal defensive response of the body. As the airway inflammation gradually improves, the amount of phlegm tends to decrease. If the phlegm is particularly viscous, treatment may include the use of medications that thin the phlegm to aid its expulsion.