Acute bronchitis is essentially an upper respiratory tract infection. Most cases of acute bronchitis are caused by a virus. The typical culprits are the same viruses that cause colds (rhinovirus) and influenza, but other viruses that can cause bronchitis include adenovirus. Occasionally, uncommon bacteria such as Mycoplasma pneumoniae and Chlamydophila pneumoniae can also cause acute bronchitis. The pathogens damage the airway cells lining the bronchi, causing inflammation and an influx of neutrophils to the cells. Neutrophils are the body's first-line responders to infection, and can quickly arrive at the site of infection. However, one side effect of neutrophils on the lining of the bronchi is hypersecretion of mucus. The excess mucus can cause an irritating feeling and provoke cough, with the sputum as the result of the mucus secreted into the airways. The symptoms of acute bronchitis, other than cough, include sore throat, runny nose, nasal congestion, malaise, and pleurisy (sharp chest pain with inspiration). Typically, acute bronchitis is seen more in children and run a benevolent course. However, acute bronchitis is seem more during winter seasons in children because its contagiousness is increased by close proximity of others with the bronchitis.

Chronic bronchitis is defined as a cough with expectorated sputum that lasts for at least three months or more for over two years. This kind of bronchitis is caused by chronic damage to the epithelial cells lining the bronchi. Recurrent insult or injury to these epithelial cells causes them to swell up and produce more mucus through a chronic inflammation cascade. In addition, cilia, which are finger-like projections that help sweep dust and other particles up the lung, are destroyed, ridding the lung of its ability to self-clean. Chronic bronchitis in most people is caused by years of smoking. The toxins in cigarette smoke are highly damaging to the cells lining the bronchi and over time, the smoke can damage the cells beyond repair. Goblet cells, which also line the bronchi, are responsible for cleaning up toxic materials such as smoke particles. However, these goblet cells also cause increased mucus secretion, leading to the wet-sounding cough in chronic bronchitis. Other less common causes of chronic bronchitis include bronchitis caused by pneumoconiosis, or inhaling dust and other particles chronically. This is often seen in coal miners and labor-based professions that are based in environments with lots of dust particles, such as mines and shipyards. Chronic bronchitis is a type of chronic pulmonary disease, and can cause misery and suffering to those afflicted by it for a lifetime.