Pneumonia is a respiratory condition in which there is inflammation of the lung.

Community-acquired pneumonia refers to 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

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 secretions from the mouth into your lungs (aspiration pneumonia)

Pneumonia caused by bacteria tends to be the most serious. 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 impaired (due to AIDS or certain medications that suppress the immune system).
• Staphylococcus aureus, Moraxella catarrhalis, Streptococcus pyogenes, Neisseria meningitidis, Klebsiella pneumoniae, or Haemophilus influenzae are other bacteria that can cause pneumonia.
• Tuberculosis can cause pneumonia in some people, especially those with a weak immune system.

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:

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