Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them.
In the United States, about 2 to 3 million people develop pneumonia each year, and 45,000 of them die. Pneumonia 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 (nursing home–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 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 oral or intravenous corticosteroids) can impair the immune system, as can the presence of diseases, such as AIDS or various types of cancer. 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. Also, a person whose immune system is impaired may not respond as well to treatment as someone whose immune system is healthy.
Other conditions that predispose 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.
Pneumonia is not a single illness but rather many different ones, each caused by a different microscopic organism—whether it is a bacterium, virus, fungus, or parasite. Usually pneumonia starts after organisms are inhaled into the lungs, but sometimes the infection is carried to the lungs by the bloodstream or organisms migrate 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. Also at risk are people who are debilitated, bedridden, paralyzed, or unconscious, because the cough reflex may be impaired or breathing may be shallow. Sometimes pneumonia occurs when particles from the mouth are inhaled and are not cleared or when an obstruction (such as a tumor blocking one of the tubes in the lungs) causes bacteria to accumulate behind the obstruction. The former type is called aspiration pneumonia, and the latter type is called obstructive pneumonia.
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.
Symptoms vary even more in infants and older people. Fever may not occur. Chest pain may not occur, or people may not be able to communicate that they have chest pain. Sometimes the only symptom is rapid breathing or a sudden refusal to eat. An older person may suddenly become confused.
A severe pneumonia can prevent oxygen from getting to the bloodstream, causing people to feel short of breath. Low levels of oxygen can be life threatening.
Some pneumonias can lead to lung abscesses (see see Abscess in the Lungs), or pus can collect around the lung, a condition called empyema (see see Types of fluid).
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 lungs with inflammatory cells and fluid, a process called consolidation. In most cases, the diagnosis of pneumonia is confirmed with a chest x-ray.
In people who are sick enough to require hospitalization, doctors often culture specimens of sputum, blood, and urine in an attempt to identify the organism causing pneumonia. They often will obtain sputum samples by giving a vapor treatment that causes the person to cough deeply (inducing sputum production) or insert a bronchoscope into the airways (see see Bronchoscopy). Sputum samples obtained by inducing a cough, particularly those obtained with a bronchoscope, are less likely to contain saliva and are more likely than expectorated sputum samples to allow doctors to identify the organism causing pneumonia. It is particularly important to identify the causative organism when people are severely ill, do not have a normal immune system, or are not responding well to treatment. However, despite these tests, the precise organism cannot be identified conclusively in most people who have pneumonia.
The most effective way to prevent pneumonia is to stop smoking. Vaccines are available that offer partial protection against pneumococcal pneumonia (caused by the bacterium Streptococcus pneumoniae) and almost 100% protection against pneumonia caused by the bacterium Haemophilus influenzae. The ability of vaccines to prevent pneumonia caused by the influenza virus depends on how well the strains used in the vaccine match the epidemic strain that occurs in a particular year. Protection has been very good in 9 of the past 10 years. Vaccination can also help prevent pneumonia caused by chickenpox.
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. People with pneumonia who are short of breath or have low levels of oxygen in their blood are given supplemental oxygen. 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, even before the organism 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 organism is likely to be the cause. Doctors can give a different antibiotic later, after the organism has been identified and its susceptibility to various antibiotics is known. Often, people who have pneumonia but are not very sick can take oral antibiotics and remain at home. Older people, infants, and those who are short of breath, are very sick, or have preexisting heart or lung disease are usually 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, if they are very sick, mechanical respiratory support (see see Acute Respiratory Distress Syndrome (ARDS)).
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.
Last full review/revision April 2008 by John G. Bartlett, MD