Bronchitis causes

Bronchitis is an inflammation or obstruction of the bronchi, the larger airways that eventually branch out to become the bronchioles (the site of asthma). It resembles asthma in some ways.

Bronchitis mostly occurs during the cold and flu season, usually coupled with an upper respiratory infection.

• Viruses
  Several viruses cause bronchitis, including influenza A and B, commonly referred to as "the flu."
• Bacteria
  Bacteria that can cause bronchitis is a bacteria called Mycoplasma pneumoniae, which causes so-called walking pneumonia, also bacteria called Klebsiella, Haemophilus.
• Bronchitis also can occur when you inhale irritating fumes or dusts. 
• Smoking
  Smoking is the leading cause of chronic bronchitis. The substances in cigarette and other forms of tobacco smoking cause irritation of the bronchial tubes. In addition, there are some early studies showing that those exposed to high enough levels of second-hand smoke may be more likely to get chronic bronchitis. There also are people with no known risk factors who still get the disease.
• People at increased risk both of getting bronchitis and of having more severe symptoms include the elderly, those with weakened immune systems, smokers, and anyone with repeated exposure to lung irritants.

Main article: Bronchitis