What is pneumonia ?

Lung infection / rxn: Any reaction in the lung that causes typical symptoms (fever, cough with phlegm, etc.) along with a visible abnormality on X-ray is considered pneumonia. Most often, it is a (bacterial) infection; however, pneumonia can be caused by a hypersensitivity (immune) reaction, viruses, exposure to harmful chemicals or inorganic material like coal dust, asbestos, & the like, radiation, & multiple others.