Community-acquired pneumonia develops in people with limited or no contact with medical institutions or settings.

• Many bacteria, viruses, and fungi can cause pneumonia.
• The most common symptom of pneumonia is a cough that produces sputum, but chest pain, chills, fever, and shortness of breath are also common.
• Doctors diagnose community-acquired pneumonia by listening to the lungs with a stethoscope and by reading x-rays of the chest.
• Antibiotics, antiviral drugs, or antifungal drugs are used depending on which organism doctors believe has caused the pneumonia.

Causes

Many organisms cause community-acquired pneumonia, including bacteria, viruses, fungi, and parasites. Causative organisms vary depending on the person's age and other factors, such as whether the person also has other disorders. The term community-acquired pneumonia is usually reserved for people who have pneumonia caused by one of the more common bacteria or viruses.

Bacteria: Streptococcus pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae, and Mycobacterium pneumoniae are the most common bacterial causes.

C. pneumoniae accounts for a small percentage of community-acquired pneumonia and is the second most common cause of lung infections in healthy people aged 5 to 35 years. C. pneumoniae is commonly responsible for outbreaks of respiratory infection within families, in college dormitories, and in military training camps. It causes a pneumonia that is rarely severe and infrequently requires hospitalization. Chlamydia psittaci pneumonia (psittacosis) is rare and occurs in people who own or are often exposed to birds.

Viruses: Viruses: Common viral causes include respiratory syncytial virus (RSV), adenoviruses, influenza viruses, metapneumovirus, and parainfluenza viruses. The virus that causes chickenpox can also cause a lung infection. Hantavirus and severe acute respiratory syndrome (SARS) are also types of lung infections. A bacterium can infect people with pneumonia originally caused by a virus.

Fungi: Common fungal causes include Histoplasma capsulatum (histoplasmosis) and Coccidioides immitis (coccidioidomycosis). Less common fungi include Blastomyces dermatitidis (blastomycosis) and Paracoccidioidesbraziliensis (paracoccidioidomycosis). Pneumocystis jiroveci commonly causes pneumonia in people who have HIV infection or are immunosuppressed.

Parasites: Parasites that cause lung infection in people who live in developed countries include Toxocara canis and T. catis (visceral larva migrans), Dirofilaria immitis (dirofilariasis), and Paragonimus westermani (paragonimiasis).

What Is Legionnaires' Disease?
Legionnaires' disease is caused by the bacterium Legionella. It accounts for about 1 to 8% of all pneumonias and about 4% of fatal pneumonias acquired in hospitals. Legionella bacteria live in water, and outbreaks have occurred primarily in hotels and hospitals when the organism has spread through the air conditioning systems or water supplies, such as showers. No cases have been identified in which one person directly infected another. Although Legionnaires' disease may occur at any age, people who are middle-aged and older have been affected most often. People who smoke tobacco, take corticosteroids, have chronic kidney failure, or have undergone organ transplantation seem to be at greater risk. Legionnaires' disease can be life threatening. The first symptoms, appearing 2 to 10 days after the infection is transmitted, include fatigue, fever, headache, and muscle aches. A dry cough later becomes productive of sputum. People with serious infections can become extremely short of breath and may have diarrhea or mental disturbances. Laboratory tests are done on sputum and urine samples to confirm the diagnosis. Because people infected with Legionella produce antibodies to fight the disease, blood tests show an increasing concentration of these antibodies. However, the results of antibody tests usually are not available until after the pneumonia has run its course. Antibiotics, such as the fluoroquinolones, erythromycinSome Trade Names E-MYCIN ERYTHROCIN , or azithromycinSome Trade Names ZITHROMAX , are used for treatment. About 20% of the people who develop the disease die. The death rate is much higher among people who contract the disease in the hospital or who have an impaired immune system.

Psittacosis: An Unusual Type of Pneumonia
Psittacosis (parrot fever) is a rare pneumonia caused by Chlamydia psittaci, a bacterium present mainly in birds such as parrots, parakeets, and lovebirds. It is also present in other birds, such as pigeons, finches, chickens, and turkeys. Usually, people are infected by inhaling dust from the feathers or the waste of infected birds. The organism also may be transmitted by a bite from an infected bird and, rarely, from person to person in cough droplets. Psittacosis mainly occurs in bird fanciers or in people who work in pet shops or on poultry farms. About 1 to 3 weeks after being infected, a person develops a fever, chills, fatigue, and loss of appetite. A cough develops, which is initially dry but later brings up greenish sputum. The fever persists for 2 to 3 weeks and then slowly subsides. The disease may be mild or severe, depending on the person's age and the extent of lung tissue involved. Blood antibody tests are the most reliable method for confirming the diagnosis, but doctors usually suspect the infection in people who have a history of exposure to birds. Bird breeders and owners can protect themselves by avoiding the dust from the feathers and the cages of sick birds. Importers are required to treat susceptible birds with a 45-day course of tetracyclineSome Trade Names SUMYCIN , which generally gets rid of the organism. People with psittacosis are treated with tetracyclineSome Trade Names SUMYCIN taken by mouth for at least 10 days. Recovery may take a long time, especially in severe cases. The death rate may reach 30% in severe untreated cases.

Symptoms

Symptoms include a general feeling of weakness (malaise), cough, shortness of breath, and chest pain. Cough typically produces sputum in older children and adults, but it is dry in infants, young children, and older people. Shortness of breath usually is mild and occurs mainly during exertion. Chest pain is typically worse when breathing in or coughing. Sometimes people have upper abdominal pain.

Symptoms vary at the extremes of age. Infants may be irritable and restless, and older people may be confused or have a decreased level of consciousness. These people may be unable to communicate chest pain and shortness of breath. Fever is common but may not occur in older people.

Diagnosis

No matter what type of pneumonia is suspected, doctors listen to a person's chest with a stethoscope to make a diagnosis. Chest x-rays are usually also done to confirm the diagnosis. Doctors usually do not need to do additional tests to determine what organism is causing the pneumonia. However, if doctors do need to identify the organism, they usually try to grow the organism from a specimen of sputum, blood, or urine. Even when such testing is done, the organism is identified less than half the time.

Sometimes the likely cause is evident from the person's symptoms. For example, a bird fancier may have psittacosis. Certain combinations of risk factors and symptoms may suggest Legionnaire's disease. In people who have symptoms typical of influenza (see Viral Infections: Influenza), influenza is a likely cause of pneumonia. In people with a rash characteristic of chickenpox and pneumonia, chickenpox is probably the cause of the pneumonia. However, a bacterium may have also infected the lung after a virus such as influenza or chickenpox first caused pneumonia.

Prevention

Stopping smoking is the best way to prevent pneumonia. Some pneumonias can be prevented by vaccination. OseltamivirSome Trade Names
TAMIFLU
or zanamivirSome Trade Names
RELENZA
can be given to prevent influenza in household contacts of people who have influenza and in people with heart or lung disorders who have not been vaccinated because these people would be at risk of severe pneumonia if they developed influenza.

Treatment

Doctors evaluate many factors to determine whether people can be safely treated at home or whether they should be hospitalized because of high risk of complications. Some of the factors include the following:

• Age
• Whether another disorder, such as cancer or a liver, heart, or lung disease, is also present
• Whether there are worrisome findings on physical examination or testing
• Whether people are able to care for themselves or have someone to help them

Antibiotics are started as soon as possible. People are also given fluids, drugs to relieve fever and pain, and supplemental oxygen if needed.

Because the causative organism is difficult to identify, doctors choose antibiotics based on the organisms that are most likely to be causing pneumonia and the severity of illness.

With antibiotic treatment, most people with bacterial pneumonia improve. In people who do not improve, doctors look for unusual organisms, resistance to the antibiotic used for treatment, infection with a second organism, or some other disorder (such as a problem with the immune system or a lung abnormality) that is delaying recovery.

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How Is Community-Acquired Pneumonia Treated? Severity Possible Drugs Comments Mild pneumonia in otherwise healthy people with no risk factors* AzithromycinSome Trade Names ZITHROMAX ClarithromycinSome Trade Names BIAXIN DoxycyclineSome Trade Names VIBRAMYCIN Drugs are taken by mouth. People are treated at home. Mild pneumonia in people with risk factors* AmoxicillinSome Trade Names AMOXIL TRIMOX AmoxicillinSome Trade Names AMOXIL TRIMOX plus clavulanate AzithromycinSome Trade Names ZITHROMAX CefpodoximeSome Trade Names VANTIN CefuroximeSome Trade Names CEFTIN ZINACEF ClarithromycinSome Trade Names BIAXIN DoxycyclineSome Trade Names VIBRAMYCIN LevofloxacinSome Trade Names LEVAQUIN QUIXIN MoxifloxacinSome Trade Names AVELOX Drugs are usually taken by mouth. Often, more than one drug is taken. People are treated at home. Moderate pneumonia or Pneumonia in people who cannot care for themselves AzithromycinSome Trade Names ZITHROMAX CefotaximeSome Trade Names CLAFORAN CeftriaxoneSome Trade Names ROCEPHIN LevofloxacinSome Trade Names LEVAQUIN QUIXIN MoxifloxacinSome Trade Names AVELOX Drugs are usually given intravenously in a hospital. Often, more than one drug is given. Severe pneumonia in people with many risk factors* Some of the same drugs used for moderate pneumonia AztreonamSome Trade Names AZACTAM CefepimeSome Trade Names MAXIPIME CiprofloxacinSome Trade Names CILOXAN CIPRO Gentamicin Imipenem MeropenemSome Trade Names MERREM Piperacillin plus tazobactam Drugs are usually given intravenously in an intensive care unit. Usually, more than one drug is given. *Risk factors include heart or lung disorders, cancer, alcoholism, age older than 65, recent use of antibiotics, and a weakened immune system (for example, because of AIDS, organ transplantation, or use of drugs that suppress the immune system).

To treat influenza pneumonia, oseltamivirSome Trade Names
TAMIFLU
or zanamivirSome Trade Names
RELENZA
can be given. To treat chickenpox pneumonia, acyclovirSome Trade Names
ZOVIRAX
is given. If a person with a viral pneumonia is very sick or does not improve within a few days after beginning treatment, doctors may prescribe antibiotics in case a bacterium has also infected the lung.

Doctors usually do follow-up chest x-rays about 6 weeks after treatment in people older than 35 to ensure that the infection has been cured.

Prognosis

Most people with community-acquired pneumonia recover. However, pneumonia can be fatal, most often in infants and in older people. Mortality is higher in Legionnaires' disease, possibly because people who develop the disease are less healthy even before they become sick.

Last full review/revision April 2008 by John G. Bartlett, MD