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Pneumonia, or inflammation of the lungs caused by infection, is often grouped into two categories: community-acquired pneumonias and hospital-acquired or nosocomial pneumonias. Community-acquired pneumonias are pneumonias that occur in usually healthy people not confined to an institution, such as a hospital or nursing home. Nosocomial pneumonias are infections that arise while a person is confined to a hospital or other institution. Each category is associated with different infection-causing microbes, called pathogens, but there are several pathogens that can cause both types of infections.

Bacterial Pneumonias
A majority of community-acquired and nosocomial pneumonias are caused by bacterial infection. Community-acquired pneumonias are most commonly caused by a type of bacteria called Streptococcus pneumoniae. These bacteria spread through coughing, sneezing, and close contact with an infected person.

There are several types of bacteria that commonly cause nosocomial pneumonias. These bacteria include Pseudomonas aeruginosaEscherichia coliEnterobacterProteus, and Klebsiella. These bacteria can be spread on the hands of healthcare workers or by equipment that gets contaminated and is not properly cleaned, and they often reach the lungs through the blood.

Pneumonia can also be caused by the bacteria that causes tuberculosis, or TB, which spreads through the air when an infected person coughs, speaks, or sings. TB bacteria can settle and begin to grow in the lungs, and from there, spread through the blood to other parts of the body, such as the kidney, spine, and brain. Pneumonia is a complication of TB in the lungs.

Aspiration Pneumonias
Aspiration pneumonias are caused by bacteria that normally live in the upper respiratory tract or the mouth, called flora, that get into the lungs. This most often occurs in patients with periodontal or gum disease, patients with reflux, alcoholics, patients with neurological disease, and patients who have a decreased or depressed level of consciousness, such as those in a coma.

Atypical Pneumonias
Atypical pneumonias are pneumonias caused by different bacteria than the more common ones. Atypical pneumonia tends to have milder symptoms than typical bacterial pneumonia. Bacteria that cause atypical pneumonia include Mycoplasma pneumoniaeChlamydophila pneumoniae, and Legionella pneumophila.

Viral Pneumonias
Viral pneumonias are rare in healthy adults, but pneumonia can be a complication of some viral infections. Viruses that can lead to pneumonia include influenza viruses, respiratory syncytial virus (RSV), adenoviruses, human metapneumovirus (HMPV), human parainfluenza viruses (HPIVs), and rhinoviruses.

Other Pneumonias
Other bacteria, viruses, and fungi rarely cause pneumonia in healthy individuals, but can cause pneumonia in patients with weakened immune systems.

Viruses that can cause pneumonia in immunocompromised patients include cytomegalovirus, herpes simplex virus, and rubeola (measles).

Bacteria that can cause pneumonia in immunocompromised patients include Actinomyces and Nocardia species.

Fungi that can cause pneumonia in immunocompromised patients in the United States include Cryptococcus neoformansSporothrixAspergillus, and Candida. There are other pneumonia-causing fungi in other parts of the world. Pneumocystis jirovecii is a fungus that is particularly well known for causing pneumonia in patients with acquired immune deficiency syndrome (AIDS).