Bronchitis is inflammation of the walls that line the tube-like bronchial passages (bronchi, bronchioles) that carry air in and out of the lungs.Inflammation of the bronchial tubes in the lungs means that the lining of the bronchials (mucous membrane) is swollen and tender, usually occurring after an acute cold or respiratory infection in the nasal passageways, sinuses, or throat. Bronchitis occurs most often as a single illness (acute bronchitis), but it can sometimes become chronic bronchitis in which the inflammation occurs several months during the year for at least two consecutive years. Smoking cigarettes or being frequently exposed to other irritants may cause acute bronchitis to develop into a chronic condition. In both acute and chronic bronchitis, inflammation is accompanied by infection, resulting in reduced airflow and causing a cough. The source of infection can be bacterial or viral. Acute bronchitis can usually be treated effectively in people who are otherwise healthy. Chronic bronchitis is a more serious and potentially long-term illness in which the individual will have a mucus-producing cough for most days in three or four months out of each year. It may act like a cold that will not clear up, but the inflammation gradually scars the lining of the bronchi and bronchioles so that mucus production is continuous and the condition develops again and again. As a result the walls that line the bronchial tubes become thicker, reducing airflow and causing constant inflammation, mucus production and coughing. This may become the permanent condition of the lungs, especially in smokers. Chronic bronchitis may also lead to emphysema, a condition that reduces the lungs’ ability to exhale air. Both chronic bronchitis and emphysema are classified as chronic obstructive pulmonary disease (COPD) and often occur together.Acute bronchitis occurs commonly among people of both sexes and all ages, with as many as 34 million doctor visits each year by people seeking care for bronchitis or upper respiratory infection (URI). Acute bronchitis develops in 60% of individuals who have the flu (influenza). More than 12 million individuals are reported to have chronic bronchitis. Among adults, chronic bronchitis occurs twice as often in men than in women and most often in smokers.Breathing (respiration) is the main function of the lungs on either side of the chest, each composed of lobes, three in the right lung and two in the left. They exchange oxygen and carbon dioxide, bringing in oxygen from the outside environment into the blood, and releasing carbon dioxide from blood that has circulated. Between the lungs is a central airway, the trachea, which then branches into the two larger bronchi that lead into each lung and divide again into smaller bronchi called bronchioles. Finally, the branching ends in small tube-like passages, the terminal bronchioles, which are composed of air-filled sacs (alveolar sacs) that contain even smaller air sacs (alveoli) that exchange oxygen and carbon dioxide through tiny blood vessels surrounding them. Moisture levels inside the lungs make it a perfect environment for bacterial growth, one of the reasons for development of bacterial and viral infections in lungs that are compromised by disease Acute bronchitis usually follows a cold or viral infection, especially flu virus, and typically lasts no more than six weeks. In acute bronchitis, passageways that are inflamed during an infection return to normal fairly quickly in normally healthy individuals after the infection is treated. However, acute bronchitis can become worse or recur if the individual smokes. The first symptom of acute bronchitis after having a cold or upper respiratory infection will be a sore throat that leads to a cough, either dry or with coughing up of sputum. Symptoms may also include tightness in the chest and some mild difficulty breathing. Usually there is no fever and symptoms subside in a week or so, except for a lingering cough. Smoking cigarettes is the most common cause of chronic bronchitis.