The causes of bronchitis include infections, physical and chemical factors, allergic reactions, smoking, and other factors. 1. Infection: Infections caused by viruses and bacteria are common. Viruses are often adenovirus, rhinovirus, herpes simplex virus, influenza virus, coronavirus, respiratory syncytial virus, and so on. Haemophilus influenzae and Streptococcus pneumoniae are common pathogenic bacteria. Chlamydia and mycoplasma infections have increased significantly in recent years. 2. Physical and chemical factors: inhalation of cold air, dust, irritant gases, etc. can stimulate the trachea – bronchial mucosa causing acute injury and inflammatory reaction. 3. Allergic reaction: pollen, organic dust, animal fur and excreta and other sensitizers into the body can also lead to the occurrence of bronchial inflammation. 4. Smoking: closely related to the occurrence of chronic bronchitis. Long-term smoking is easy to cause the inhibition of cilia movement on the bronchial mucosa; it can also lead to bronchospasm. 5. Other factors: autonomic dysfunction, aging, abnormal immune function, nutrition may also have a certain relationship with the occurrence of bronchitis. If you have any discomfort symptoms, you need to go to the hospital as soon as possible, the doctor according to the specific condition, make individualized diagnosis and treatment plan, so as not to delay the condition.