 [ Symptoms ] what is pneumonia [ Disease ] pneumonia is an infection of one or both lungs which is usually caused by bacteria viruses or fungi prior to the discovery of antibiotics one third of all people who developed pneumonia subsequently died from the infection currently over 3 million people develop pneumonia each year in the united states over a half a million of these people are admitted to a hospital for treatment although most of these people recover approximately 5 will die from pneumonia pneumonia is the sixth leading cause of death in the united states