Pneumonia refers to an infection of the lungs. Lobar pneumonia is usually restricted to a single lobe of one lung (although more than one lobe may be involved), while bronchial pneumonia affects more widespread areas in both lungs. The speciﬁc type of pneumonia depends on the agent causing the infec- tion. The three major microorganisms that cause pneumonia are viruses, bacte- ria, and mycoplasmas.
About half of all cases of pneumonia are caused by viruses. This type of pneu- monia is most common in infants and children, older adults, and people whose immune systems are not working effectively. Viral pneumonia has symptoms similar to those of the ﬂu: fever, headache, muscle pain, weakness, dry cough, and breathlessness. Medications are available to combat some of the viruses that can cause pneumonia, but antibiotics should not be prescribed, and would not be beneﬁcial, unless bacteria are causing the infection.
Bacterial infection accounts for 30 to 50 percent of pneumonia cases in adults. Streptococcus pneumoniae (pneumococcus) is the most common bacterium involved, although many other microorganisms can cause bacterial pneumonia. The bacteria that cause pneumonia can spread throughout the body once they have entered the lungs. This can result in infection in the bloodstream (bac- teremia or sepsis), the covering of the brain (meningitis), the lining of the heart (endocarditis), or the ﬂuid in the joints (septic arthritis). Symptoms of pneumo- coccal pneumonia can appear either suddenly or gradually and include fever, pain on the affected side, shortness of breath, and a cough that produces mucus (the mucus is often blood-streaked).
Another type of bacterial pneumonia, called legionnaires’ disease, is caused by the Legionella pneumophila bacterium. The natural habitats for these bacteria are bodies of water, but they also thrive in the evaporative condensers of air- conditioning systems and may be found in humidiﬁers and vaporizers as well. Legionnaires’ disease is most common among middle-aged men. Risk factors include smoking, alcohol abuse, and a suppressed immune system (especially due to taking corticosteroid medications). The fever associated with legion- naires’ disease is usually high, and other ﬂulike symptoms occur, such as a vague sense of being ill, a cough, muscle pain, and a headache. The cough is initially dry but produces more mucus as the disease progresses. Antibiotics will elimi- nate the bacteria, but recovery may be slow.
Mycoplasmas are microorganisms that have characteristics of both bacteria and viruses. They tend to cause a mild but widespread form of pneumonia. Mycoplasma pneumonia is most common among children and young adults, especially those in closed communities such as schools, military barracks, and families. This microorganism acts by attaching to and destroying the cilia throughout the airways. Early symptoms (such as a vague sense of being ill, sore throat, and a dry cough) resemble the ﬂu, but gradually, violent coughing bouts develop. Most people recover without treatment, although the use of certain antibiotics can speed recovery in some cases.
Pneumonia also can develop if bacteria, food, or other substances (including liquids or vomit) are inhaled (aspirated) directly into the lungs. This may occur when a person is choking or unconscious. The resultant infection is called aspi- ration pneumonia. Because aspiration pneumonia can cause severe damage to the lungs, it is treated in the hospital with intravenous antibiotics and supple- mental oxygen. If food or other substances remain lodged in the lungs, the doc- tor may need to perform a bronchoscopy (see “Diagnostic Procedures,” page 256) to remove them. If a toxic chemical has been inhaled, it is a medical emer-
gency. Call 911 or your local emergency number, or take the person to the near- est hospital emergency department without delay.
Who Should Be Vaccinated against Pneumonia?
Pneumonia causes about 40,000 deaths in the United States each year. You should have a “pneumonia shot” if:
• you are age 65 or older
• you have diabetes
• you have chronic heart, lung, kidney, or liver disease
• you have sickle-cell disease
• your immune system is compromised (due to cancer, HIV, or AIDS, or from corticos- teroid medication, radiation therapy, or chemotherapy)
• you have had your spleen removed or have a spleen dysfunction
• you have had an organ transplant or a bone marrow transplant
• you are a healthcare worker
The pneumococcal vaccine provides long-term immunity and can be given at any time of year. You should be revaccinated if you received the vaccine in childhood and have a chronic condition such as sickle-cell disease.