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 fourth most frequent cause of hospitalizations.
Although the majority of pneumonias respond well to treatment, the infection kills 40,000 - 70,000 people each year.
Men with community-acquired pneumonia tend to fare worse than women. Men are 30% more likely than women to die from the condition, even if the severity of the illness is the same. Researchers say there may be some genetic reason for the disparity.
Outlook for High-Risk Individuals
Hospitalized Patients. The death rate for community-acquired pneumonia can range from less than 5% in mildly ill outpatients to 10 - 30% in patients who need to be admitted to a hospital. If pneumonia develops in patients already hospitalized for other conditions, or those in a nursing home, death rates can be much higher. This is especially true for anyone who is on a ventilator.
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 An Impaired Immune System. Pneumonia is particularly serious in people with an impaired immune system. 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 spleen has 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 take time. Severe and life-threatening cases of pneumonia are more likely to occur in elderly people with other medical conditions.
- Streptococcus pneumoniae is the most common cause of pneumonia, as well as all other 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 can spread easily and cause widespread damage. Treatments have improved dramatically since it was first identified. However, many patients may experience serious problems, including coughing, shortness of breath, fatigue, and nervous system (neurological) and muscle 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 is more commonly seen with 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 the danger. Drainage with a needle may also be needed. 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 more severe 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 physical changes in the lungs that make it even harder for the lungs to exchange oxygen (ventilatory 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 is a two-layered membrane that surrounds each lung.
In some cases of pneumonia the pleura become inflamed, which can result in breathlessness and 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. At least one study has also linked bacterial pneumonia with an increased risk of acute heart problems, such as heart attack or abnormal heart rhythm (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. Hydration through a vein (intravenous) controls the problem.
Long-Term Effects of Atypical Pneumonias
Pneumonias cased by the atypical organisms mycoplasma and chlamydia are usually mild.
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Asthma. Chlamydia pneumoniae, Mycoplasma pneumoniae, and RSV are becoming suspects in many cases of severe adult asthma. One small study found evidence of previous chlamydia infection in 64% of the patients with asthma who were tested.