Bronchitis occurs most often during the cold and flu season, usually coupled with an upper respiratory infection.
Several viruses cause bronchitis, including influenza A and B, which we commonly call "the flu."
A number of bacteria are known to cause bronchitis, such as Mycoplasma pneumoniae, which causes so-called walking pneumonia.
Bronchitis also can occur when you inhale irritating fumes or dusts. Chemical solvents and smoke, including tobacco smoke, have been linked to acute bronchitis.
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.