Community-acquired pneumonia is lung infection that develops in people who are not patients in 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.
Antibiotic, antiviral, or antifungal medications are used depending on which organism doctors believe has caused the pneumonia.
The airways and small air sacs of the lungs are constantly exposed to microorganisms. The nose and throat are full of bacteria and sometimes viruses, and people regularly inhale from the air or aspirate from the digestive tract, mouth, or throat small amounts of these organisms. Normally these organisms are readily dealt with by the lungs' defense mechanisms. However, if the defense mechanisms are not functioning correctly, pneumonia can develop.
In the United States, community-acquired pneumonia results in > 1.5 million hospitalizations annually. The risk of death as a result of pneumonia has declined in recent decades. However, pneumonia remains a leading cause of death in the United States; approximately 1 of 3 people hospitalized with community-acquired pneumonia die within 1 year.
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.
Community-acquired pneumonia can develop in people whose immune systems are working normally (also called competent) or in people whose immune systems are impaired (also called immunocompromised people). However, when doctors use the term community-acquired pneumonia, they are generally referring to people whose immune systems are normal. Pneumonia in immunocompromised people is discussed separately.
"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.
Bacterial causes of pneumonia
The most common bacterial causes of community-acquired pneumonia are:
Streptococcus pneumoniae
Hemophilus influenzae
Staphylococcus aureus
Gram negative bacteria (eg, Enterobacterales, Pseudomonas)
Less typical organisms causing pneumonia include:
Chlamydia pneumoniae
Mycoplasma pneumoniae
Legionella pneumophila
Streptococcus pneumoniae (pneumococcus) causes many hospitalizations in the United States each year. Pneumococcal pneumonia can be very severe, particularly in young children and older adults and in people who do not have a spleen (asplenia). Streptococcus pneumoniae is still the most common bacterial infection causing pneumonia but its prevalence is decreasing in the United States and other resource-rich countries because of vaccination.
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.
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. Doctors suspect it in people with an influenza infection that is not resolving (as a secondary infection). S. aureus pneumonia can result in lung cavities, localized destruction of the lung tissue, and infected fluid surrounding the lungs.
Chlamydophila pneumoniae is the second most common cause of lung infections in healthy people (both children and adults). 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 years, 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. Mycoplasma is a common cause of "walking" pneumonia (pneumonia that allows people to continue daily activities).
Legionella pneumophila causes pneumonia and flu-like symptoms sometimes called Legionnaires’ disease. 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.
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.
Q fever, tularemia, anthrax, and plague are uncommon bacterial causes of pneumonia that can occur in humans subsequent to exposure to infected animals or animal products. In particular, tularemia, anthrax, and plague may raise concerns about bioterrorism.
For information on related disease in animals, see Q Fever in Cats, Tularemia in Animals, Anthrax in Dogs, and Plague in Animals.
Viral causes of pneumonia
Common viruses that can cause community-acquired pneumonia include:
Influenza viruses
Rhinovirus (common cold virus)
Adenoviruses
Epstein-Barr virus (rarely)
Coxsackievirus (rarely)
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:
Histoplasma capsulatum (histoplasmosis)
Coccidioides immitis (coccidioidomycosis)
Less common fungal causes of pneumonia include Blastomyces dermatitidis (blastomycosis) and Paracoccidioides braziliensis (paracoccidioidomycosis). Pneumocystis jirovecii commonly causes pneumonia in people who have human immunodeficiency virus (HIV) infection or a weakened immune system.
Parasites that cause pneumonia
Parasites that rarely cause community-acquired pneumonia in people include:
Toxocara canis or T. catis (toxocariasis)
Dirofilaria immitis (dirofilariasis)
Paragonimus westermani (paragonimiasis)
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 adults. 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, restlessness, and decreases in appetite and urination. Older adults may be confused or have a decreased level of consciousness. Older adults and young children may be unable to communicate chest pain and shortness of breath. Fever is common but may not occur in older adults.
Diagnosis of Community-Acquired Pneumonia
Usually a chest x-ray or ultrasound 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, ultrasound scans, 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 (such as in people whose condition does not improve while taking initial antibiotics), they usually try to grow or identify parts of 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.
Treatment of Community-Acquired Pneumonia
Antibiotics
Sometimes antiviral or antifungal medications
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 ). People are also given fluids, medications to relieve fever and pain, and oxygen if needed. People who smoke should be offered smoking cessation advice and treatment.
Because the causative organism is difficult to identify, doctors choose antibiotics based on the organisms that are most likely to be causing pneumonia.
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 lungs (for example, into the lining of the lungs [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 medications oseltamivir, zanamivir, peramivir, or baloxavir can be given. To treat chickenpox pneumonia, acyclovir is given. To treat RSV pneumonia in people who are very sick, ribavirin may be 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 lungs.
To treat fungal pneumonia, antifungal medications may be given.
Sometimes steroids (sometimes called glucocorticoids or corticosteroids) may be given to some people being treated for severe forms of pneumonia.
People with pneumonia should follow up with their primary care doctor. 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 adults to ensure that the abnormal findings seen on chest x-ray represent pneumonia only rather than an underlying cancer with pneumonia.
How Is Bacterial Community-Acquired Pneumonia Treated in Adults?
Severity | Possible Medications | Comments |
|---|---|---|
Mild pneumonia in otherwise healthy people with no risk factors* | Amoxicillin Doxycycline Azithromycin Clarithromycin | Medications are taken by mouth. People are treated at home. |
Mild pneumonia in people with risk factors* | Amoxicillin Amoxicillin/clavulanic acid Cefpodoxime Cefuroxime Azithromycin Clarithromycin Doxycycline Levofloxacin Moxifloxacin | Medications are usually taken by mouth. Often, more than one medication is taken. People are treated at home. |
Moderate pneumonia or Pneumonia in people who cannot care for themselves | Cefotaxime Ceftriaxone Azithromycin Levofloxacin Moxifloxacin | Medications are usually given intravenously in a hospital. Often, more than one medication is given. |
Severe pneumonia in people with many risk factors* | Some of the same medications used for moderate pneumonia: Aztreonam Cefepime Ciprofloxacin Imipenem/cilastatin Linezolid Vancomycin Meropenem Piperacillin/tazobactam | Medications are usually given intravenously in an intensive care unit. Usually, more than one medication is given. |
* Risk factors include heart or lung disorders, cancer, alcohol use disorder, age older than 65 years, recent use of antibiotics, and a weakened immune system (for example, because of HIV, organ transplantation, or use of medications that suppress the immune system). | ||
Prognosis for Community-Acquired Pneumonia
Most people with community-acquired pneumonia recover. However, pneumonia can be fatal, most often in infants and in older adults.
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 medications oseltamivir or zanamivir to prevent influenza.
Vaccines to prevent pneumonia
Vaccines are available that offer partial protection against pneumonia caused by:
The bacterium Streptococcus pneumoniae
Haemophilus influenzae type b (only in children)
Chickenpox (varicella)
Drug Information for the Topic
