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). S. 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:

• aspiration pneumonia
• atypical pneumonia
• bacterial pneumonia
• CMV pneumonia
• hospital-acquired pneumonia
• Legionella pneumonia
• Mycoplasma pneumonia (walking pneumonia)
• necrotizing pneumonia
• pediatric pneumonia
• Pneumocystis carinii pneumonia
• pneumonia in immunocompromised host
• pneumonia with lung abscess
• pyogenic pneumonia
• viral pneumonia

• cough with mucus-like, greenish, or pus-like sputum
• chills with shaking
• fever
• easy fatigue
• chest pain
  • sharp or stabbing
  • increased by deep breathing
  • increased by coughing
• headache
• loss of appetite
• nausea and vomiting
• general discomfort, uneasiness, or ill feeling (malaise)
• joint stiffness (rare)
• muscular stiffness (rare)

• shortness of breath
• excessive sweating
• clammy skin
• nasal flaring
• coughing up blood
• rapid breathing
• anxiety, stress, and tension
• abdominal pain

• chest X-ray
• Gram's stain and culture of the sputum
• CBC
• arterial blood gases

• thoracic CT
• routine sputum culture
• pulmonary ventilation/perfusion scan
• pleural fluid culture
• lung needle biopsy

• acuterespiratory failure
• death
• empyema

Call your health care provider if symptoms suggesting pneumonia develop.

Vaccination can help prevent some types of pneumonia in children and at-risk adults (such as the elderly). These include:

• pneumococcal vaccine (Pneumovax (R) (prevents pneumonia caused by Streptococcus pneumoniae)
• flu vaccine
• Hib vaccine (prevents pneumonia from Haemophilus influenzae type b)

Coughing and deep breathing may help prevent some forms of hospital-acquired pneumonia. See also the prevention steps listed under specific types of pneumonia.