Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs (alveoli)—associated with fever, chest symptoms, and a lack of air space (consolidation) on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes. Infectious agents include: bacteria, viruses, fungi, and parasites.

Typical symptoms include cough, chest pain, fever, and difficulty breathing. Diagnostic tools include x-rays and examination of the sputum. Vaccines to prevent certain types of pneumonia are available. Treatment depends on the underlying cause. Presumed bacterial pneumonia is treated with antibiotics.

Although pneumonia was regarded by William Osler in the 19th century as "the captain of the men of death", the advent of antibiotic therapy and vaccines in the 20th century have seen radical improvements in survival outcomes. Nevertheless, in the third world, and among the very old, the very young and the chronically ill, pneumonia remains a leading cause of death.

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