What causes pneumonia?

Scanning Electron Micrograph of Streptococcus pneumoniae.

Pneumonia can be caused by bacteria and viruses. Sometimes fungi can also be the cause (1), (2). Because pneumonia can have over 30 different causes, it has been classified according to the type of germ that cause it and to place where you got the infection (1).

Why classification is important?
Even though, the community-acquired pneumonia, and the health-acquired pneumonia can be caused by the same germ, the one caught in the hospital or health-care facility is usually a lot more serious. This is because hospitals are the source of antibiotic-resistant strains of germs.

Those with viral infections may be secondarily infected with the bacteria Streptococcus pneumoniae, Staphylococcus aureus, or Haemophilus influenzae, particularly when other health problems are present.