Pneumonia is an infection of the air sacs and surrounding lung tissue. It is the seventh leading cause of death and the primary cause of death from infectious disease in the United States, claiming the lives of more than 56,000 Americans each year. Most people who die of pneumonia are over age 65, and often they have had underlying disorders that increased their susceptibility to infection.
Based on x-ray results, the various types of pneumonia are often divided into those that affect a single lobe (lobar pneumonia) and those that occur as patches in several lobes (multilobar pneumonia). In addition, pneumonia may involve either one lung (unilateral) or both lungs (bilateral).