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Community-Acquired Pneumonia

by John G. Bartlett, MD

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.


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.


Streptococcus pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae, and Mycoplasma 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: 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.


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


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).


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.


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 Influenza (Flu)), 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.


Stopping smoking is the best way to prevent pneumonia. Some pneumonias can be prevented by vaccination. Oseltamivir or zanamivir 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.


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.

To treat influenza pneumonia, oseltamivir or zanamivir can be given. To treat chickenpox pneumonia, acyclovir 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.

How Is Community-Acquired Pneumonia Treated?


Possible Drugs


Mild pneumonia in otherwise healthy people with no risk factors*




Drugs are taken by mouth.

People are treated at home.

Mild pneumonia in people with risk factors*


Amoxicillin plus clavulanate








Drugs are usually taken by mouth.

Often, more than one drug is taken.

People are treated at home.

Moderate pneumonia


Pneumonia in people who cannot care for themselves






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







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).


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.

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