Pneumonia is an inflammatory condition of the lung affecting primarily the microscopic air sacs known as alveoli. It is usually caused by infection with viruses or bacteria and less commonly other microorganisms, certain drugs and other conditions such as autoimmune diseases.

Typical symptoms include a cough, chest pain, fever, and difficulty breathing. Diagnostic tools include x-rays and culture 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. If the pneumonia is severe, the affected person is generally admitted to hospital.

People with infectious pneumonia often have a productive cough, fever accompanied by shaking chills, shortness of breath, sharp or stabbing chest pain during deep breaths, and increased respiratory rate. In the elderly, confusion may be the most prominent sign. The typical signs and symptoms in children under five are fever, cough, and fast or difficult breathing.

Fever is not very specific, as it occurs in many other common illnesses, and may be absent in those with severe disease or malnutrition. In addition, a cough is frequently absent in children less than two months old. More severe signs and symptoms may include blue tinged skin, decreased thirst, convulsions, persistent vomiting, extremes of temperature, or a decreased level of consciousness.