What is the difference between pneumonia and severe bronchitis?

Negative cxr: Bronchitis without any cxr evidence of infiltrate.

Location...: Bronchitis is an infection of the airways whereas pneumonia is an infection of the lung parenchyma itself. Clinically, pneumonia tends to be more severe than bronchitis but there is plenty of overlap so this distinction cannot always be made on clinical grounds alone. A chest x-ray will usually tell the difference as pneumonia usually shows on a chest x-ray but bronchitis does not show.