Definition of pneumonia
: an acute disease that is marked by inflammation of lung tissue accompanied by infiltration of alveoli and often bronchioles with white blood cells (such as neutrophils) and fibrinous exudate, is characterized by fever, chills, cough, difficulty in breathing, fatigue, chest pain, and reduced lung expansion, and is typically caused by an infectious agent (such as a bacterium, virus, or fungus) — see bronchopneumonia, pneumocystis carinii pneumonia, primary atypical pneumonia — compare pneumonitis
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Merriam-Webster unabridged
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Time Traveler for pneumonia
The first known use of pneumonia was in 1603
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