Bronchitis is an inflammation of all or part of the bronchial tree (the bronchi), through which air passes into the lungs. During the passage through the bronchi, microorganisms and other foreign bodies are removed from the air by tiny hair like structures called cilia, which project from the cells that line the bronchial wall. These cilia have a wavelike motion and sweep the foreign material upward toward the trachea and larynx. Because of this irritation a thick mucus is produced by glands in the bronchial wall and aid in the elimination of the foreign material. Such material and the secreted mucus stimulate nerve endings in the bronchial wall and cause you to cough in an effort to expel the foreign material.
Acute bronchitis is caused by any of a great number of agents and not as a specific disease. It is most frequently caused by viruses responsible for upper respiratory infections and is, therefore, often part f what is called the common cold.