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 Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more 
.)
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 normal (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. 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 Streptococcal Infections Streptococcal infections are caused by any one of several species of Streptococcus. These gram-positive, sphere-shaped (coccal) bacteria (see figure ) cause many disorders, including... read more (pneumococcus) causes about 150,000 hospitalizations 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 Haemophilus influenzae Infections Haemophilus influenzae are gram-negative bacteria that can cause infection in the respiratory tract, which can spread to other organs. Infection is spread through sneezing, coughing,... read more 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 Chronic Obstructive Pulmonary Disease (COPD) Chronic obstructive pulmonary disease is persistent narrowing (blocking, or obstruction) of the airways occurring with emphysema, chronic obstructive bronchitis, or both disorders. Cigarette... read more 
(COPD) and bronchiectasis Bronchiectasis Bronchiectasis is an irreversible widening (dilation) of portions of the breathing tubes or airways (bronchi) resulting from damage to the airway wall. The most common cause is severe or repeated... read more .
Chlamydophila pneumoniae Chlamydia and Other Nongonococcal Infections Chlamydial infections include sexually transmitted infections of the urethra, cervix, and rectum that are caused by the bacteria Chlamydia trachomatis. These bacteria can also infect... read more 
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 Psittacosis: An Unusual Type of Pneumonia 
) 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 Chlamydia and Other Nongonococcal Infections Chlamydial infections include sexually transmitted infections of the urethra, cervix, and rectum that are caused by the bacteria Chlamydia trachomatis. These bacteria can also infect... read more 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 Legionella Infections Legionella infections are caused by the gram-negative bacteria Legionella pneumophila and most often affect the lungs, causing pneumonia and flu-like symptoms. The infection is... read more 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 Staphylococcus aureus Infections Staphylococcus aureus is the most dangerous of all of the many common staphylococcal bacteria. These gram-positive, sphere-shaped (coccal) bacteria (see figure ) often cause skin infections... read more 
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 Pseudomonas Infections Any of several types of the gram-negative bacteria Pseudomonas, especially Pseudomonas aeruginosa, can infect different parts of the body, particularly in people who have serious... read more is an especially common cause of pneumonia in people with cystic fibrosis Cystic Fibrosis (CF) Cystic fibrosis is a hereditary disease that causes certain glands to produce abnormally thick secretions, resulting in tissue and organ damage, especially in the lungs and the digestive tract... read more 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
SARS-CoV-2, the virus that causes COVID-19 Coronaviruses and Acute Respiratory Syndromes (MERS and SARS) Coronaviruses are a large family of viruses that cause respiratory illness ranging in severity from the common cold to fatal pneumonia. There are many different coronaviruses. Most of them cause... read more
Respiratory syncytial virus Respiratory Syncytial Virus (RSV) Infection and Human Metapneumovirus Infection Respiratory syncytial virus infection and human metapneumovirus infection cause upper and sometimes lower respiratory tract infections. Respiratory syncytial virus is a very common cause of... read more (RSV)
Adenoviruses
Parainfluenza viruses
Coxsackievirus (rarely)
The virus that causes chickenpox Chickenpox Chickenpox is a highly contagious viral infection with the varicella-zoster virus that causes a characteristic itchy rash, consisting of small, raised, blistered, or crusted spots. Chickenpox... read more 
can also cause a lung infection. Hantavirus Hantavirus Infection Hantavirus infection is a viral disease that is spread from rodents to people. The virus can cause severe infections of the lungs (with cough and shortness of breath) or kidneys (with abdominal... read more and severe acute respiratory syndrome Severe Acute Respiratory Syndrome (SARS) Coronaviruses are a large family of viruses that cause respiratory illness ranging in severity from the common cold to fatal pneumonia. There are many different coronaviruses. Most of them cause... read more (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 fungal causes of pneumonia include Blastomyces dermatitidis (blastomycosis Blastomycosis Blastomycosis is an infection, mainly of the lungs, caused by the fungus Blastomyces dermatitidis. People have a fever, chills, and drenching sweats and sometimes chest pain, difficulty... read more 
) and Paracoccidioides braziliensis (paracoccidioidomycosis Paracoccidioidomycosis Paracoccidioidomycosis is an infection caused by the fungus Paracoccidioides brasiliensis. Spores of the fungus, which grow in soil, may be inhaled. Most people have no symptoms, but... read more ). Pneumocystis jirovecii commonly causes pneumonia in people who have human immunodeficiency virus (HIV) infection Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and is treated with antiretroviral medications. If untreated, it can cause... read more 
or a weakened immune system (immunocompromise Pneumonia in Immunocompromised People Pneumonia is infection of the lungs. Pneumonia in people whose immune system is weakened or impaired (for example, by acquired immunodeficiency syndrome [AIDS], cancer, organ transplantation... read more ).
Parasites that cause pneumonia
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
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 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 Symptoms Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more or of chickenpox Symptoms Chickenpox is a highly contagious viral infection with the varicella-zoster virus that causes a characteristic itchy rash, consisting of small, raised, blistered, or crusted spots. Chickenpox... read more , 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 for 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 Smoking Cessation Most people who smoke want to quit and have tried doing so with limited success. Effective tools to help quit smoking include counseling, nicotine replacement products, and medications. While... read more is the best way to prevent pneumonia Prevention Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more .
Some pneumonias can be prevented by vaccination Vaccines to prevent pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more . 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? 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.
