Pneumonia is an acute inflammatory condition of lung parenchyma (lung tissue excluding the airways) caused by a variety of infectious agents and toxins and favored by aspects of the environment and/or the general physical status of the patient. The term “pneumonia” is derived from the Greek word περιπλευμονιη meaning “condition about the lung”; the word refers to a clinicopathological state that arises in several different yet specific disease patterns. All of these are characterized by some degree of fever, cough, chest pain, and difficulty in breathing. Technically speaking, pneumonitis, which means “inflammation of the lung,” is a synonym for pneumonia, but the former is usually reserved for benign, localized, and sometimes chronic inflammation without major toxemia (generalized effects). Many modifiers and eponyms are applied to the term pneumonia to reflect the cause (e.g., embolic pneumonia) or the localization (e.g., pleuro- or broncho pneumonia). The classic form is lobar pneumonia, an infectious but not particularly contagious condition usually localized to part or all of one of the five lobes of the lungs, and caused by a pneumococcus, the gram-positive organism Streptococcus pneumoniae (formerly called Diplococcus pneumoniae). Untreated lobar pneumonia has a mortality of about 30 percent, but the advent of antibiotic treatment has improved survival rates.

Several other pathogens (bacterial, viral, fungal, and parasitic) are recognized causative agents. The extent of the pulmonary involvement; the onset, pattern, and duration of symptoms; as well as the mortality rate depend on both the causative organism and precipitating factors. Chemical irritation, environmental exposure to noxious substances, or hypersensitivity can occasionally cause pneumonia.