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Pneumonia is a very common, often serious illness that affects about 3-4 million people each year in the United States. Many different organisms can cause pneumonia, including bacteria, viruses, and fungi. Bacterial pneumonias tend to be the most serious. In adults, bacteria are the most common cause, and of these Streptococcus pneumoniae (pneumococcus) is the most common.

Respiratory viruses are the most common causes of pneumonia in young children, peaking between the ages of 2 and 3. By school age, the bacterium Mycoplasma pneumoniae becomes more common.

In some people, particularly the elderly and those who are debilitated, pneumonia may follow influenza.

Pneumonia is classified according to a number of different systems. One classification system divides pneumonia into "community-acquired" (pneumonia contracted outside of a health-care institution) and "hospital-acquired" (acquired in a hospital or other long-term health care facility). Streptococcus pneumoniae is the most common form of community-acquired pneumonia.

Hospital-acquired pneumonia tends to be more serious because the patient's defense mechanisms against infection are often impaired when someone is hospitalized, due to the condition that initially required treatment . In addition, there is a greater possibility of infection with bacteria that are resistant to antibiotics.

Other classifications of pneumonia include: