Pneumonia pathogen is an infection in one or both lungs. It may involve just one small section of your lung, or it may encompass your entire lung. The pathogen cause the lung’s air sacs to become inflamed and filled with pus, fluid, and cellular debris. Pneumonia pathogen can be mild or serious. It may even lead to respiratory failure or death. The severity of your pneumonia depends on the strength of the bacteria and how quickly you are diagnosed and treated. Your age, health, and immune status are also factors.