Pneumonia is an inflammatory illness of the lung. Frequently, it is described as lung parenchyma/alveolar inflammation and abnormal alveolar filling with fluid (consolidation and exudation).

The alveoli are microscopic air-filled sacs in the lungs responsible for absorbing oxygen. Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, or parasites, and chemical or physical injury to the lungs. Its cause may also be officially described as idiopathic—that is, unknown—when infectious causes have been excluded.

Typical symptoms associated with pneumonia include cough, chest pain, fever, and difficulty in breathing. Diagnostic tools include x-rays and examination of the sputum. Treatment depends on the cause of pneumonia; bacterial pneumonia is treated with antibiotics.

Pneumonia is a common illness which occurs in all age groups, and is a leading cause of death among the elderly and people who are chronically and terminally ill. Additionally, it is the leading cause of death in children under five years old worldwide. Vaccines to prevent certain types of pneumonia are available. The prognosis depends on the type of pneumonia, the appropriate treatment, any complications, and the person’s underlying health.