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Community-Acquired Pneumonia


Sanjay Sethi

, MD, University at Buffalo SUNY

Last full review/revision Dec 2020| Content last modified Dec 2020

Community-acquired pneumonia is lung infection that develops in people outside a hospital.

  • 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 or computed tomography (CT) scans of the chest.
  • Antibiotics, antiviral drugs, or antifungal drugs are used depending on which organism doctors believe has caused the pneumonia.

(See also Overview of Pneumonia.)

Causes of Community-Acquired Pneumonia

Many organisms cause community-acquired pneumonia, including bacteria, viruses, fungi, and parasites. In most cases, the specific microorganism causing the pneumonia is not identified. However, doctors can usually predict which microorganisms are most likely to be causing the pneumonia based on the person’s age and other factors, such as whether the person also has other diseases. The term community-acquired pneumonia is usually reserved for people who have pneumonia caused by one of the more common bacteria or viruses.

"Walking pneumonia" is a nonmedical term used to describe a mild case of community-acquired pneumonia that does not require bedrest or hospitalization. Some people even feel well enough to go to work and participate in other daily activities. The cause is often a viral lung infection or a bacterial infection with Mycoplasma pneumoniae or Chlamydophila pneumoniae.

Bacterial causes of pneumonia

The most common bacterial causes of community-acquired pneumonia are

  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Chlamydophila pneumoniae
  • Mycoplasma pneumoniae

Streptococcus pneumoniae (pneumococcus) causes about 900,000 cases of pneumonia in the United States each year. There are over 90 types of pneumococci, but most serious disease is caused by only a small number of types. Pneumococcal pneumonia can be very severe, particularly in young children and older people.

Haemophilus influenzae pneumonia may occur in adults but is more common among children. However, childhood infection has become much less common since children have been routinely vaccinated against H. influenzae. H. influenzae pneumonia is more common among adults who have underlying chronic lung disorders such as chronic obstructive pulmonary disease (COPD) and bronchiectasis.

Chlamydophila pneumoniae 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 a rare infection caused by a different strain of chlamydia and occurs in people who own or are often exposed to birds.

Mycoplasma pneumoniae causes infection very similar to that caused by C. pneumoniae. M. pneumoniae pneumonia is more common among older children and adults younger than 40, especially those living in crowded environments, such as schools, college dormitories, and military barracks. Although the illness is rarely severe, symptoms can last for weeks or even months.

Legionella pneumophila causes pneumonia and flu-like symptoms sometimes called Legionnaires’ disease. 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.

Staphylococcus aureus causes pneumonia that is resistant to some types of antibiotics. This bacteria is known as community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), and it can cause severe pneumonia, primarily in young adults. Since the year 2000, the number of cases of community-acquired pneumonia caused by S. aureus has increased, but the infection is still uncommon.

Pseudomonas aeruginosa is an especially common cause of pneumonia in people with cystic fibrosis and other lung diseases and in those with an impaired immune system.

Viral causes of pneumonia

Common viruses that can cause community-acquired pneumonia include

The virus that causes chickenpox can also cause a lung infection. Hantavirus and severe acute respiratory syndrome (SARS) are also viral causes of pneumonia. Sometimes a bacterial pneumonia can occur after a lung infection caused by a virus.

Fungal causes of pneumonia

Fungal causes of community-acquired pneumonia include

Less common fungi include Blastomyces dermatitidis (blastomycosis) and Paracoccidioides braziliensis (paracoccidioidomycosis). Pneumocystis jirovecii commonly causes pneumonia in people who have human immunodeficiency virus (HIV) infection or are immunocompromised.

Parasites that cause pneumonia

Parasites that cause community-acquired pneumonia in people who live in developed countries include

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, macaws, 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 (thick or discolored mucus). 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 tests to determine whether the person has a certain type of antibody, suggesting recent infection with Chlamydia psittaci, 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 dust from the feathers and cages of sick birds. Importers are required to treat susceptible birds with a 45-day course of tetracycline, which generally gets rid of the organism.

People with psittacosis are treated with tetracycline 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 people with severe untreated psittacosis, although with appropriate treatment most people recover.

Symptoms of Community-Acquired Pneumonia

Symptoms of community-acquired pneumonia include

  • A general feeling of weakness (malaise)
  • Cough
  • Shortness of breath
  • Fever
  • Chills
  • Chest pain

Cough typically produces sputum (thick or discolored mucus) in older children and adults, but the cough is dry in infants, young children, and older people. Shortness of breath usually is mild and occurs mainly during activity. Chest pain is typically worse when breathing in or coughing. Sometimes people have upper abdominal pain.

Symptoms vary at the extremes of age. Symptoms of pneumonia in infants and toddlers may include irritability and restlessness. Older people may be confused or have a decreased level of consciousness. Older people and young children may be unable to communicate chest pain and shortness of breath. Fever is common but may not occur in older people.

Diagnosis of Community-Acquired Pneumonia

  • A doctor's examination
  • Usually a chest x-ray or computed tomography (CT) scan of the chest

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 or CT scans 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, because most organisms are effectively treated by standard treatment and identifying the organism does not make a difference in treatment success. 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 person's symptoms or risk factors may suggest the cause of the pneumonia. For example, a bird fancier may have psittacosis. So doctors may ask detailed questions about a person's work, hobbies, or recent travel. Certain combinations of risk factors and symptoms may suggest Legionella infection. In people who have first had symptoms typical of influenza or of chickenpox, most pneumonia that develops is actually a bacterial pneumonia that took root in the lungs inflamed by the virus. However, sometimes influenza and chickenpox cause pneumonia directly.

Prognosis of Community-Acquired Pneumonia

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

Prevention of Community-Acquired Pneumonia

Stopping smoking is the best way to prevent pneumonia.

Some pneumonias can be prevented by vaccination. Unvaccinated people who are at high risk of severe pneumonia and who are in close contact with a person who has influenza can be given the antiviral drugs oseltamivir or zanamivir to prevent influenza.

Treatment of Community-Acquired Pneumonia

  • Antibiotics
  • Sometimes antiviral drugs or antifungal drugs

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 disorder, 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. Doctors may prescribe antibiotics based on the severity of the infection and the risk of complications (see table How Is Community-Acquired Pneumonia Treated?). People are also given fluids, drugs to relieve fever and pain, and 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 an alternative diagnosis, unusual organisms, resistance to the antibiotic used for treatment, infection with a second organism, spread of the infection beyond the lung (for example, into the lining of the lung [pleura]), or some other disorder (such as a problem with the immune system or a blocked (obstructed) airway) that is delaying recovery.

To treat influenza pneumonia, the antiviral drugs oseltamivir, zanamivir, or baloxavir can be given. To treat chickenpox pneumonia, acyclovir is given. If a person with a presumed viral pneumonia is very sick or does not improve within a few days after beginning treatment, doctors may prescribe antibiotics in case bacteria have also infected the lung.

To treat fungal pneumonia, antifungal drugs may be given.

Sometimes doctors do follow-up chest x-rays about 6 weeks after treatment to make sure that any abnormal findings on chest x-ray have resolved. Follow-up may be more important in people who smoke and in older people to ensure that the abnormal findings seen on chest x-ray represent pneumonia only rather than an underlying cancer with pneumonia.

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




Imipenem plus cilastatin


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).

Drugs Mentioned In This Article

Generic Name Select Brand Names
Clarithromycin BIAXIN
Ciprofloxacin CILOXAN, CIPRO
Azithromycin ZITHROMAX
tetracycline ACHROMYCIN V
Moxifloxacin AVELOX
oseltamivir TAMIFLU
Amoxicillin AMOXIL
Ceftriaxone ROCEPHIN
Cefpodoxime No US brand name
Cefotaxime CLAFORAN
acyclovir ZOVIRAX
Meropenem MERREM
zanamivir RELENZA
Aztreonam AZACTAM

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