In the early 20th century, pneumonia was a leading cause of death in the United States. Antibiotics knocked it down the list, but pneumonia is still dangerous.
Pneumonia develops when the lungs become infected or inflamed. The affected tissue produces pus, blocking the airways. Oxygen can't get from the lungs to the bloodstream, starving the whole body. If the infection spreads to the bloodstream, it can invade several organs.
Pneumonia comes in many forms. It may be caused by bacteria, fungi, viruses or chemicals. Altogether, more than 30 causes of pneumonia are known.
Young, healthy people who seek treatment quickly have the best chance of a fast and complete recovery. Older people or people with other illnesses or a weakened immune system may develop a more severe infection that's harder to treat.