Introduction
Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them.
In the United States, about 2 million people develop pneumonia each year, and 40,000 to 70,000 of them die. Often, pneumonia is the final illness in people who have other serious, chronic diseases. It is the sixth most common cause of death overall, and the most common fatal infection acquired in hospitals. In developing countries, pneumonia is either the leading cause of death or second only to dehydration from severe diarrhea.
The setting in which pneumonia develops is one of the most important features to doctors. Pneumonia may develop in people living in the community (community-acquired pneumonia), in the hospital (hospital-acquired pneumonia), or in some other institutional setting, such as a nursing home (institution-acquired pneumonia). The setting often helps determine what infecting organism is responsible for the pneumonia. For example, community-acquired pneumonia is more likely to stem from infection with the gram-positive bacterium Streptococcus pneumoniae. Hospital-acquired pneumonia is more likely to be caused by Staphylococcus aureus or a gram-negative bacterium, such as Klebsiella pneumoniae or Pseudomonas aeruginosa. Depending on the infecting organism, there is usually a difference in the severity of pneumonia and the way it is treated (for example, whether with oral drugs at home or with intravenous drugs in the hospital).
Another critical feature is whether the pneumonia occurs in a healthy person or in someone who has an impaired immune system. Certain drugs (such as corticosteroids) can impair the immune system, as can the presence of diseases, such as AIDS. Sometimes the immune system can be worn down by a severe acute or chronic illness, as is often the case with older people. A person who has an impaired immune system is far more likely to contract pneumonia, including pneumonia caused by unusual organisms; this person may not respond as well to treatment as someone whose immune system is healthy. Other conditions that predispose certain people to pneumonia include alcoholism, cigarette smoking, diabetes, heart failure, and chronic obstructive pulmonary disease. The very young and very old are at higher-than-average risk. Also at risk are people who are debilitated, bedridden, paralyzed, or unconscious.
Causes
Pneumonia is not a single illness but rather many different ones, each caused by a different microscopic organism--whether it is a bacterium, virus, or fungus. Usually pneumonia starts after organisms are inhaled into the lungs, but sometimes the infection is carried to the lungs by the bloodstream or it migrates to the lungs directly from a nearby infection. Pneumonia may follow surgery, particularly abdominal surgery, or an injury (trauma), particularly a chest injury, because of the resulting shallow breathing, impaired ability to cough, and retention of mucus. Sometimes pneumonia occurs when particles from the mouth are inhaled and are not cleared, or when an obstruction (such as a tumor) causes bacteria to become trapped. The former type is called aspiration pneumonia; the latter type is called obstructive pneumonia.
Symptoms and Diagnosis
The most common symptom of pneumonia is a cough that produces sputum. Other common symptoms include chest pain, chills, fever, and shortness of breath. These symptoms may vary, however, depending on how extensive the disease is and which organism is causing it.
A doctor or nurse checks for pneumonia by listening to the chest with a stethoscope. Pneumonia usually produces distinctive sounds; these abnormal sounds are caused by narrowing of airways or filling of the normally air-filled parts of the lung with inflammatory cells and fluid, a process called consolidation.
In most cases, the diagnosis of pneumonia is confirmed with a chest x-ray. For most bacterial pneumonias, the involved tissue of the lung appears on the x-ray as a dense white patch (because the x-ray beam does not get through), compared with nearby healthy lung tissue that appears black (because the x-rays get through easily, exposing the film). Viral pneumonias typically produce faint, widely scattered white streaks or patches. Some pneumonias can lead to a lung abscess (see Section 4, Chapter 43), which appears on the x-ray as a space filled with fluid (pus). There may be changes at the top of the lung, suggesting tuberculosis (see Section 17, Chapter 193). Thus, the x-ray may (but not always) help a doctor determine which organism is causing the disease.
Doctors culture sputum and blood specimens in an attempt to identify the organism causing pneumonia. However, despite these tests, the precise organism cannot be identified in up to half of people who have pneumonia. When it is necessary to identify the organism, such as when the person is severely ill and is not responding well to therapy, doctors can try to obtain better specimens by inserting a bronchoscope into the airways (bronchoscopy (see Section 4, Chapter 39)).
Prevention and Treatment
Several types of pneumonia can be prevented with the use of vaccines. Vaccines are available to protect against pneumococcal pneumonia, pneumonia caused by the bacterium Haemophilus influenzae, and pneumonia caused by the influenza virus, which also often leads to a secondary bacterial pneumonia.
Deep-breathing exercises and therapy to clear secretions help prevent pneumonia in people at high risk, such as those who have had chest or abdominal surgery and those who are debilitated.
People with pneumonia also need to clear secretions and benefit from deep-breathing exercises and therapy as well. If people with pneumonia are short of breath or their blood is low in oxygen, supplemental oxygen is provided. Although rest is an important part of treatment, moving often and getting out of bed and into a chair are encouraged.
Usually antibiotics are started whenever bacterial pneumonia is suspected (including obstructive pneumonia), even before the bacteria is identified. The prompt use of antibiotics likely reduces the severity of pneumonia and the chance of developing complications, some of which can lead to death.
When choosing an antibiotic, doctors consider which bacteria is likely to be the cause. The doctor can change the choice of antibiotic later, after the bacteria has been identified and its susceptibility to various antibiotics known. Often, people who have pneumonia but are not very sick can take oral antibiotics and remain at home. Older people and those who are short of breath or have preexisting heart or lung disease are generally hospitalized and given intravenous antibiotics to start. Those antibiotics are usually switched to oral ones after a few days. These people may also need supplemental oxygen, intravenous fluids, and mechanical respiratory support (see Section 4, Chapter 55).
Antibiotics are not helpful for viral pneumonias. However, antibiotics are given for viral pneumonias that are likely to be followed by bacterial infections, such as those caused by respiratory syncytial virus infection in infants and sometimes those caused by the influenza virus, at least in some people who are very susceptible to pneumonia.
 See the sidebar Preventing Certain Pneumonias With Vaccines.
See the sidebar Antibiotics Used for Bacterial Pneumonias.
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