Prognosis:

Between 5 and 10 million people get pneumonia in the United States each year, and more than 1 million people are hospitalized due to the condition. As a result, pneumonia is the third most frequent cause of hospitalizations (births are first, and heart disease is second). About 500,000 children are hospitalized for respiratory infections each year, and a third of them have pneumonia.

Although the majority of pneumonias respond well to treatment, the infection kills 40,000 - 70,000 people each year.

Outlook for High-Risk Individuals

Hospitalized Patients. For patients who need hospitalization for pneumonia, the death rate is 10 - 25%. If pneumonia develops in patients already hospitalized for other conditions, death rates range from 50 - 70%, and are higher in women than in men.

Older Adults. Community-acquired pneumonia is responsible for 350,000 - 620,000 hospitalizations in the elderly every year. Older adults have lower survival rates than younger people. Even when older individuals recover from CAP, they have higher-than-normal death rates over the next several years. Elderly people who live in nursing homes or who are already sick are at particular risk.

Very Young Children. Small children who develop pneumonia and survive are at risk for developing lung problems in adulthood, including chronic obstructive pulmonary disease (COPD). Research suggests that men with a history of pneumonia and other respiratory illnesses in childhood are more than twice as likely to die of COPD as those without a history of childhood respiratory disease.

Pregnant Women. Pneumonia poses a special hazard for pregnant women, possibly due to changes in a pregnant woman's immune system. This complication can lead to premature labor and increases the risk of death during pregnancy.

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

Patients With Serious Medical Conditions. Pneumonia is also very dangerous in people with diabetes, cirrhosis, sickle cell disease, cancer, and in those whose spleens have been 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 potent (virulent) but 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 medical conditions.
• Streptococcus pneumoniae is the most common cause of pneumonia and, in fact, all bacterial upper respiratory infections. It can produce severe pneumonia, with mortality rates of 10%. Nevertheless, pneumococcal pneumonia responds very well to many antibiotics.
• Staphylococcus aureus is a Gram-positive bacterium that often causes severe pneumonia in hospitalized and high-risk patients and following influenza A and B. People who get this form of pneumonia may develop pockets of infection in their lungs (abscesses) that are difficult to treat and can cause the death of lung tissue (necrosis). Mortality rates are 30 - 40%, in part because the patients who develop this infection are generally very ill or vulnerable.
• Pseudomonas aeruginosa and Klebsiella pneumoniae 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 study suggested that many patients experience long-term problems, including coughing, shortness of breath, fatigue, and neurological and muscular complications.
• Viral pneumonia is usually very mild, but there are exceptions. Respiratory syncytial virus (RSV) pneumonia rarely poses a danger for healthy young adults, but it can be life-threatening in infants and serious in the elderly.

Complications of Pneumonia

Abscess. An abscess in the lung is a thick-walled, pus-filled cavity that forms when infection has destroyed lung tissue. It typically occurs as a result of aspiration pneumonia, when a mixture of organisms is carried into the lung. Untreated abscesses can cause hemorrhage (bleeding) in the lung, but targeted antibiotic therapy significantly reduces their danger. Abscesses are more common with Staphylococcus aureus, Pseudomonas aeruginosa, or Klebsiella pneumoniae, and are uncommon with Streptococcus pneumoniae.

Respiratory Failure. Respiratory failure is one of the top 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 (ventilatory failure) or oxygen loss in the arteries (hypoxemic respiratory failure).

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

Pleural Effusions and Empyema. The pleura are two thin membranes that line the chest and lungs:

• 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.

In about 20% of pneumonia cases fluid builds up between the pleural membranes, a condition known as pleural effusion. Ordinarily, the narrow zone between the two membranes contains only a tiny amount of fluid, which lubricates the lungs.

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. This is called pneumothorax. It may be a complication of pneumonia (particularly Streptococcus pneumoniae ) or of the invasive procedures used to treat pleural effusion.

Pneumothorax occurs when air leaks from inside the lung to the space between the lung and the chest wall. The lung then collapses. The dark side of the chest (right side of the picture) fills with air from outside of the lung tissue.

Other Complications of Pneumonia. In rare cases, infection may spread from the lungs to the heart and possibly throughout the body. This can cause abscesses in the brain and other organs. Coughing up blood (hemoptysis) is another potentially serious complication of pneumonia, particularly in patients with lung problems such as cystic fibrosis. At least one study has also linked bacterial pneumonia with an increased risk of acute heart problems, such as heart attack or arrhythmia.

Kidney complications and electrolyte imbalances are common in patients admitted to the hospital with pneumonia. If not treated, these problems cause more severe illness and increase the risk of death. Treatment with intravenous saline can usually resolve the problem.

Long-Term Effects of Atypical Pneumonias

Pneumonias cased by the atypical organisms mycoplasma and chlamydia are usually mild. Some research suggests, however, that chlamydia may have powerful inflammatory effects in the blood vessels. This effect may have certain adverse long-term consequences, even in healthy younger individuals.

Asthma. Chlamydia pneumoniae, Mycoplasma pneumoniae, and RSV are becoming suspects in many cases of severe adult asthma. One small Australian study found evidence of previous chlamydia infection in 64% of the patients with asthma who were tested.