Definition of Pneumonia:

Pneumonia is a breathing (respiratory) condition in which there is an infection of the lung.

Community-acquired pneumonia is pneumonia in people who have not recently been in the hospital or another health care facility (nursing home, rehabilitation facility).

See also: Hospital-acquired pneumonia

Causes, incidence, and risk factors:

Pneumonia is a common illness that affects millions of people each year in the United States. Germs called bacteria, viruses, and fungi may cause pneumonia.

Ways you can get pneumonia include:

• Bacteria and viruses living in your nose, sinuses, or mouth may spread to your lungs.
• You may breathe some of these germs directly into your lungs.
• You breathe in (inhale) food, liquids, vomit, or fluids from the mouth into your lungs (aspiration pneumonia)

Pneumonia caused by bacteria tends to be the most serious kind. In adults, bacteria are the most common cause of pneumonia.

• The most common pneumonia-causing germ in adults is Streptococcus pneumoniae (pneumococcus).
• Atypical pneumonia, often called walking pneumonia, is caused by bacteria such as Legionella pneumophila, Mycoplasma pneumoniae, and Chlamydophila pneumoniae.
• Pneumocystis jiroveci pneumonia is sometimes seen in people whose immune system is not working well.

Many other bacteria can also cause pneumonia.

Viruses are also a common cause of pneumonia, especially in infants and young children.

See also: Respiratory syncytial virus

Risk factors (conditions that increase your chances of getting pneumonia) include:

• Cerebral palsy
• Chronic lung disease (COPD, bronchiectasis, cystic fibrosis)
• Cigarette smoking
• Difficulty swallowing (due to stroke, dementia, Parkinson's disease, or other neurological conditions)
• Immune system problem (See also: Pneumonia in immunocompromised host)
• Impaired consciousness (loss of brain function due to dementia, stroke, or other neurologic conditions)
• Living in a nursing facility
• Other serious illnesses, such as heart disease, liver cirrhosis, or diabetes mellitus
• Recent surgery or trauma
• Recent cold, laryngitis, or flu