Pneumonia is an infection in the lungs that can be caused by a variety of organisms, including bacteria, viruses, and fungi. These infections are characterized by inflammation and/or a buildup of fluid and pus in the alveoli – the tiny air sacs of the lungs. Pneumonia can, in many cases, become very serious, resulting in extreme sickness or even death. While anyone can be affected by pneumonia, the condition is most dangerous in people over age 65, children under the age of 5, and individuals who are affected by underlying health problems or have weakened immune systems.