Pneumonia describes the condition where the air sacs in the lungs that normally are filled with air fill up with fluid. The fluid is usually a response by the body's immune system to the presence of an infection in the lungs in an attempt to overwhelm the infection. However, the fluid prevents the lungs from taking in air, leading to a rapid loss of lung capacity and, if untreated, eventual death.
Pneumonia is often the immediate cause of death for persons with respiratory diseases, although the condition always has an underlying cause.
Even until the 1960s, pneumonia could develop rapidly and would almost always result in the death of the patient. However, the use of respirators now allow physicians to keep the patient breathing until the body can recover.