Author prahalad singh explains: Definition: Pneumonia is an acute or chronic disease marked by inflammation of one or both lungs. An inflammation of the lungs caused by viruses, bacteria, or other microorganisms and sometimes by physical and chemical irritants. The air sacs in the lungs fill with pus and other liquid. Oxygen has trouble reaching your blood. If there is too little oxygen in your blood, your body cells can't work properly. Because of this and spreading infection through the body pneumonia can cause death.