Pneumonia is a major lung infection that is caused by a virus, bacterium, or fungus. It can be mild, serious, or even fatal in its impact. An estimated 4 million people develop pneumonia in the United States annually, and despite ready access to antibiotics for most Americans, 1.2 million patients are hospitalized for pneumonia each year, according to the Centers for Disease Control and Prevention. About 52,000 people in the United States die each year of pneumonia. Pneumonia explains the causes, symptoms, diagnosis, and treatment of this serious disease.