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An in-depth report on the causes, diagnosis, treatment, and prevention of pneumonia.

Alternative Names

Antibiotics; Bronchitis: Acute


About 1.2 million people are hospitalized each year for pneumonia, which is the third most frequent reason for hospitalizations (births are first and heart disease is second). Although the majority of pneumonias respond well to treatment, the infection can still be a very serious problem. It kills between 40,000 and 70,000 people each year. Together with influenza, pneumonia is the sixth leading cause of death in the US and is the leading cause of death from infection.

Outlook for High-Risk Individuals

Hospitalized Patients. For patients who require hospitalization for pneumonia, the mortality rate is between 10% and 25%. If pneumonia develops in patients already hospitalized for other conditions, the mortality rates are higher. They range from 50% to 70% and are greater in women than in men.

Older Adults. Community-acquired pneumonia is responsible for 350,000 to 620,000 hospitalizations in elderly people every year. The elderly have lower survival rates than younger people, and pneumonia and influenza are the fifth leading causes of death in this population. Even when older individuals recover from community-acquired pneumonia, they have higher than normal mortality rates over the next several years. Elderly people at particular risk are those with other medical problems and nursing home patients.

Very Young Children. About 20% of deaths in stillborn and very young infants are due to pneumonia. Small children who develop pneumonia and survive are at also at risk for developing lung problems in adulthood.

Pregnant Women. Pneumonia poses a special hazard for pregnant women.

Patients With Impaired Immune Systems. Pneumonia is particularly serious in people with impaired immune systems, particularly AIDS patients, in whom pneumonia causes about half of all deaths.

Patients With Serious Medical Conditions. The disease is also very dangerous in people with diabetes, cirrhosis, sickle cell disease, cancer, and in those who have had their spleens removed.

Risk by Organisms

Specific organisms vary in their effects. Mild pneumonia is usually associated with the atypical organisms Mycoplasma and Chlamydia. Severe pneumonia is most often associated with a wide range of organisms. Some are very virulent (potent) but are extremely curable, while others are difficult to treat.

  • Mycoplasma and Chlamydia are the most common causes of mild pneumonias and are most likely to occur in children and young adults. They rarely require hospitalization when they are appropriately treated, although recovery may still be prolonged. Severe and life-threatening cases are more likely to occur in elderly people with other medication conditions.
  • S. Pneumonia is the most common cause of pneumonia and, in fact, all upper respiratory infections. It can produce severe pneumonia, with mortality rates of 10%. Nevertheless, it is very responsive to many antibiotics.
  • Staphylococcus aureus is a gram-positive bacterium that often causes severe pneumonia in hospitalized patients and following influenza A and B in high-risk patients. People who get this form of pneumonia may develop pockets of infection in their lungs called abscesses that are difficult to treat and can cause death of lung tissue, or necrosis. Mortality rates are high, 30% to 40%, in part because the patients who develop this infection are generally very ill or vulnerable.
  • Pseudomonas aeruginosa and Klebsiella pneumonia are gram-negative bacteria that pose a risk for abscesses and severe lung tissue damage.
  • Legionella pneumophila is very virulent and can cause widespread damage. Treatments have improved dramatically since it was first identified. However, a 2002 suggested that many patients experience long-term problems, including neurologic and muscular complications, coughing, shortness of breath, and fatigue.
  • Viral pneumonia is usually very mild, but there are exceptions. Pneumonia associated with influenza can be serious. Respiratory syncytial virus (RSV) pneumonia rarely poses a danger for healthy young adults, but can be life threatening in infants and serious in the elderly. The incidence seems to be increasing.

Complications of Pneumonia

Abscesses. Abscesses in the lung are thick-walled, pus-filled cavities that are formed when infection has destroyed lung tissue. They are frequently a result of aspiration pneumonia if a mixture of organisms is carried into the lung. Abscesses can cause hemorrhage (bleeding) in the lung if untreated, but antibiotics that target them have significantly reduced their danger. Abscesses are more common with Staphylococcus aureus or Klebsiella pneumoniae, and uncommon with Streptococcus pneumoniae.

Respiratory Failure. Respiratory failure is one of the important causes of death in patients with pneumococcal pneumonia. Acute respiratory distress syndrome (ARDS) is the specific condition that occurs when the lungs are unable to function and oxygen is so severely reduced that the patient's life is at risk. Failure can occur if pneumonia leads to mechanical changes in the lungs (called ventilatory failure) or oxygen loss in the arteries (called hypoxemic respiratory failure).

Bacteremia. Bacteremia (bacteria in the blood) is the most common complication of pneumococcus infection, although it rarely spread to others sites. Bacteremia is a frequent complication of infection from other gram-negative organisms, including Haemophilus influenzae.

Pleural Effusions and Empyema. The pleura are two thin membranes:

  • The visceral pleura cover the lungs.
  • The parietal pleura cover the chest wall.

In some cases of pneumonia, the pleura become inflamed, which can result in breathlessness and acute chest pain when breathing.

Also, in about 20% of pneumonia cases there is build-up of the fluid between the pleural membranes that lubricates the lung. (Ordinarily the narrow zone between the two membranes contains only a tiny amount of fluid.)

In most cases, particularly in Streptococcus pneumoniae, the fluid remains sterile (no bacteria are present), but occasionally it can become infected and even filled with pus (a condition called empyema). Empyema is more likely to occur with specific organisms, such as Staphylococcus aureus or Klebsiella pneumoniae infections. The condition can cause permanent scarring.

Collapsed Lung. In some cases, air may fill up the area between the pleural membranes causing the lungs to collapse, a condition called pneumothorax. It can be a complication of pneumonia (particularly Streptococcus pneumoniae) or of some of the invasive procedures used to treat pleural effusion.

Other Complications of Pneumonia. In rare cases, infection may spread from the lungs to the heart and can even spread throughout the body, sometimes causing abscesses in the brain and other organs. Severe hemoptysis (coughing up blood) is another potentially serious complication of pneumonia, particularly in patients with other lung problems such as cystic fibrosis.

Long Term Effects of Atypical Pneumonias

The pneumonias cased by the atypical organisms Mycoplasma and Chlamydia are usually mild. Some research is suggesting, however, that these organisms, particularly Chlamydia, may have very powerful inflammatory affects in blood vessels, which can theoretically have certain adverse long-term effects even in healthy younger individuals.

Heart Disease and Stroke. Research has suggested that the C. pneumoniae may trigger the immune system to react. This may cause inflammation and damage over time in the arteries of the heart and elsewhere, a process called atherosclerosis or hardening of the arteries. Atherosclerosis can lead to heart attacks and strokes. Studies on a causal relationship between C. pneumonia and heart disease have been mixed. The most recent ones have found no strong association between the infection and heart disease while others underplay a possible link.

C. pneumoniae has been associated with a thickening in the carotid artery that leads to the brain, which is a risk factor for stroke. As with heart disease, however, it is not clear whether the organisms pose any significant risk for stroke.

Asthma. Chlamydia pneumoniae, Mycoplasma pneumoniae, and the respiratory syncytial virus are becoming important suspects in many cases of severe adult asthma. One small Australian study, for example, found evidence of previous C. pneumoniae infection in 64% of the asthmatic patients tested. (Serious respiratory infections that occur in early childhood, however, probably do not play a role in asthma that develops in adulthood.)


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