Pneumonia is an inflammation of the lungs, usually caused by an infection. It is very common in the US—about 2 million Americans develop pneumonia each year and upwards of 70,000 people die from it, especially those over age 65 or who have an immune problem such as AIDS.

Pneumonia is most often caused by bacteria or viruses, but can also be due to a fungus or a parasite. Many types of pneumonia can be treated effectively with antibiotics.

Doctors often divide pneumonia into two main types, depending on whether the person caught it while living at home (community-acquired pneumonia) or while staying in a hospital or nursing home (hospital-acquired pneumonia). The types of bacteria responsible for pneumonia that someone catches in the community tend to be different, and often less severe, from those that cause hospital-acquired pneumonia.