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What Is Pneumonia? 

Pneumonia is an infection of one or both lungs. Viruses, bacteria, or fungi can cause the infection. When a person contracts pneumonia, their alveoli, or air sacs in the lungs, become congested with fluid or pus. This can lead to symptoms such as cough, fever, or chills, all of which can vary in severity. A person’s health and age and the source of the infection can all affect the severity of their symptoms.1

Pneumonia affects people of all ages, but rates of the disease tend to increase with age in the U.S.2,3 Each year, the country reports nearly 25 cases of pneumonia for every 10,000 adults. Among infectious diseases, pneumonia is the eighth leading cause of death in the U.S.3 It led to 1.4 million U.S. emergency department visits in 2021 and more than 47,000 deaths in the U.S in 2020.4,5

Following good hygiene practices, such as handwashing and disinfecting high-touch surfaces, can help prevent pneumonia.5

Pneumonia is classified according to its causes, which can include bacteria, viruses, and fungi.6

Bacterial pneumonia is pneumonia that results from an overgrowth of bacteria in the lungs. Four types of bacterial pneumonia exist:7

  • Community-acquired pneumonia (CAP): An infection arising after exposure to bacteria as a person goes about their daily life outside a hospital or healthcare setting.
  • Hospital-acquired pneumonia: Pneumonia that arises after 48 hours of a person staying in a hospital. The person did not require breathing interventions while in the hospital.
  • Ventilator-associated pneumonia: Pneumonia that develops 48 or more hours after a patient requires breathing assistance, such as a ventilator or intubation.
  • Healthcare-acquired pneumonia: This type of pneumonia arises in other types of healthcare facilities, such as outpatient clinics, nursing homes, or dialysis centers, within three months of a visit.

Some common bacteria responsible for bacterial pneumonia include Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae, among others. When these common bacteria cause pneumonia, the resulting pneumonia is called typical pneumonia.7

Less common bacteria, such as Mycoplasma pneumoniae, Legionella, and Chlamydia psittaci, can cause atypical pneumonia.7

Viral pneumonia is pneumonia that arises when a virus infects the lungs. Many common viruses can cause viral pneumonia, including respiratory syncytial virus (RSV), rhinovirus (the common cold), influenza, and coronaviruses like COVID-19, among others.8

Specific groups of people are more susceptible to viral pneumonia. The infection is more common in older adults and young children. Pregnant women are also more susceptible to viral pneumonia, as are people with weakened immune systems from diseases like HIV/AIDS or from certain medications and treatments.8


Fungal pneumonia occurs when a fungus infects the lungs. Common sources of fungal pneumonia in the U.S. include Coccidioides, Histoplasma, and Blastomyces. Cases of fungal pneumonia tend to be more prevalent in people who have weakened immune systems, including people with HIV/AIDS and those who have received organ transplants, among others.9

Aspiration pneumonia: This type of pneumonia occurs when fluids that shouldn’t be in the lungs enter the lungs and cause infection. Fluids from the nasal passages or stomach contents can cause aspiration pneumonia. This is a serious condition with a mortality rate that can be as high as 70%, depending on how much foreign material enters the lungs.10

Walking pneumonia: While not a clinical classification of pneumonia, this informal term describes pneumonia that does not require treatment in the hospital or bed rest. However, walking pneumonia can be serious and may cause symptoms such as headache, cough, chills, fever, and chest pain. The person may spread the bacteria or virus to others. People with walking pneumonia tend to feel better after three to five days, but symptoms, such as cough, can last longer. Over-the-counter treatments may provide symptom relief.6,11

Prevalence of Pneumonia

Pneumonia is common in the U.S., with approximately 25 cases occurring in every 10,000 adults.3 In 2021, pneumonia sent about 1.4 million people to emergency departments in the U.S.; in 2020, it caused more than 47,000 deaths.5

Researchers estimate that community-acquired pneumonia (pneumonia that does not originate in hospitals, rehab facilities, or nursing homes) kills an estimated 3 million people around the world each year.7,12 In one global study, researchers found that cases of pneumonia sent 6.8 million older adults to the hospital. The same study found that pneumonia resulted in 1.1 million deaths among those hospitalized patients.13

Age is one of the primary factors that affects the prevalence of pneumonia. While people of any age can contract pneumonia, it tends to predominantly affect the young and the old. Children age 2 or younger face an increased risk of pneumonia because their immune systems are still maturing. Premature infants face even greater risks.14

According to the World Health Organization (WHO), pneumonia killed more than 800,000 children younger than 5 in a single year. The same year, pneumonia caused 15% of total deaths in the same age group.15

Adults age 65 and up also face an increased pneumonia risk, stemming from age-related declines in the immune system. People in this age group also tend to have other health conditions that make pneumonia a more serious threat.14

Data show that rates of mortality vary among racial and ethnic groups. For example, one study involving 30 of the largest U.S. cities found that mortality rates from influenza and pneumonia were 16% higher for Black people than for white people.16

Causes and Risk Factors

What Causes Pneumonia?

Bacteria, viruses, and fungi can all cause pneumonia.6

Virus background
  • Bacterial pneumonia can begin after a cold or flu, or it can start directly as a bacterial infection. It tends to infect a singular part, or lobe, of the lung.6 

    Common bacterial causes of pneumonia include:6

    • Streptococcus pneumoniae (pneumococcus): These bacteria typically reside in the upper respiratory tract. It causes nearly an estimated 1 million infections in the U.S. annually. When this type of bacteria causes pneumonia, it’s called pneumococcal pneumonia, which is the most common type of bacterial pneumonia.
    • Mycoplasma pneumonia: This type of bacteria typically causes pneumonia among people under age 40. It also tends to arise among people who live or work in crowded spaces.
    • Chlamydophila pneumoniae: This germ causes several respiratory ailments throughout the year, including pneumonia.
    • Legionella pneumophila: Exposure to water that has been contaminated with these bacteria can lead to pneumonia. It is not passed between people. The resulting pneumonia, called Legionnaire’s disease, is hazardous.
  • Many viruses that lead to upper respiratory tract infections can also cause pneumonia. In most cases, viral pneumonia tends to be less serious and pass more quickly than bacterial pneumonia. Sometimes bacterial pneumonia can follow viral pneumonia.6

    Common viral causes of pneumonia include:

    • SARS-CoV-2: This is the virus that leads to COVID-19. Pneumonia resulting from COVID-19 can cause a serious condition in which levels of oxygen in the blood dip dangerously low, inducing respiratory shutdown. This pneumonia also tends to affect both lungs, causing them to fill with fluid and making it difficult to breathe. It may take months to recover from pneumonia caused by SARS-CoV-2.6
    • Influenza: The seasonal influenza virus is most likely to cause viral pneumonia among adults. Pneumonia resulting from influenza can be serious and deadly. People who are pregnant and people who have pre-existing diseases of the heart or lungs face the greatest risks from influenza-related pneumonia.6
    • Respiratory syncytial virus (RSV): This virus is the top cause of children’s viral pneumonia.6 Symptoms of RSV infection often resemble a mild cold, but the virus poses a serious health risk for children and certain older adults.17 Among children under age 1, RSV is the leading cause of pneumonia.18
  • Fungal pneumonia tends to affect people with weakened immune systems and those with chronic health issues. It can also arise in people who encounter high quantities of specific types of fungi in bird droppings or soil.6

    Common fungal causes of pneumonia include:

    • Pneumocystis jirovecii: This fungus can cause pneumonia in people who take medications that suppress the immune system, including those used for cancer treatment or following an organ transplant. It can also arise in people with a weakened immune system because of HIV/AIDS. Healthy adults may have pneumocystis jirovecii in their lungs, but their immune systems prevent the fungus from causing illness. Even if they do not have symptoms, when they breathe, they can exhale the fungus, which can then infect others.6,19
    • Coccidioides: This fungus, which in the U.S. resides in the Southwestern deserts and Southern California, can also cause valley fever. Valley fever occurs when someone or something disturbs soil containing the fungus, sending fungal spores airborne. People nearby may inhale the spores and get sick. The resulting infection may require medication or resolve on its own. People with weakened immune systems, people who are pregnant, people who have diabetes, and people who are Black or Filipino face greater risks of severe infection.6,20,21
    • Histoplasma capsulatum: Abundant in the soil of the Mississippi and Ohio River valleys, this fungus becomes airborne during disturbances to the soil containing it. If a person inhales the airborne fungal spores, they may contract a type of pneumonia called histoplasmosis. People with compromised immune systems face heightened risks of severe histoplasmosis.6,22  
    • Cryptococcus neoformans: Widespread across the U.S., this fungus stems from bird droppings and feces-contaminated soil and could cause pneumonia when inhaled. Most Cryptococcus neoformans exposures do not result in infections. They tend to occur in those with compromised immune systems, particularly people with serious cases of HIV/AIDS. Cryptococcus neoformans is not spread from person to person. Infection results from environmental exposure.6,23,24
  • Some types of viruses and bacteria that cause pneumonia can be spread, while others cannot. They spread when people inhale respiratory droplets that are airborne because of coughing, talking, or sneezing. People also can contract pneumonia by touching a contaminated surface and then their mouth or nose. Exposure to the viruses, bacteria, or fungi that cause pneumonia does not mean you will get infected.6

Pneumonia Risk Factors 

Certain characteristics and circumstances can put people at an increased risk for pneumonia. They include age, environment, lifestyle, and pre-existing medical conditions.14

  • Children ages 2 or younger face an increased risk of pneumonia because their immune systems are still developing. Babies who are born prematurely face an even greater risk of pneumonia.14

    People ages 65 and older also face greater risk of pneumonia. The immune system tends to weaken with age, and people ages 65 and older are more likely to have other health conditions that raise their risk.14 

  • Where someone lives or works can affect their risk of pneumonia. For example, people living in crowded spaces, such as nursing homes, military barracks, homeless shelters, or prisons, face a higher pneumonia risk. Being hospitalized or placed on a ventilator also heightens pneumonia risk.14

    Those who work in places where exposure to toxic fumes or air pollution is common may face greater odds of getting pneumonia. That is also true of people who work in veterinary clinics, pet shops, and poultry facilities, around birds or other animals.14

  • Smoking cigarettes and using drugs or alcohol can increase a person’s risk of pneumonia.14

  • Medical conditions that weaken the immune system, such as HIV/AIDS, transplants of bone marrow or organs, and pregnancy, all raise a person’s risk of pneumonia. The use of steroid medications or chemotherapy, which weaken the immune system, also increases the risk of pneumonia.14

    Lung diseases such as COPD, cystic fibrosis, and asthma heighten the risk of pneumonia, along with other chronic conditions, such as diabetes, heart disease, kidney disease, and liver disease.14

    Finally, any medical condition that makes it difficult to swallow or cough can amplify someone’s odds of getting pneumonia. The problem occurs when food, saliva, or drink enters the windpipe instead of the esophagus.14

Pneumonia Prevention

People can take steps to lower their risk of developing pneumonia. Maintaining basic health habits may lower pneumonia risk, including washing hands frequently, quitting smoking, staying active and fit, and following a healthy diet.25

Another preventive step for eligible adults is vaccination. Adult vaccines may help guard against bacteria and viruses that cause pneumonia. Adult vaccines may also help avoid pneumonia complications, severe infections, and longer-lasting cases of illness.25

Pneumococcal vaccines target the most common cause of bacterial pneumonia: Streptococcus pneumoniae.6,25 These vaccines are appropriate for people ages 65 and older and people ages 2 and under. They are also appropriate for those with weakened immune systems, people with conditions that increase the risk of infection by pneumonia-causing bacteria, and people who smoke.25

Pneumonia in Children

According to the World Health Organization (WHO), pneumonia causes 14% of all deaths of all children under the age of 5. It claimed the lives of 740,180 children worldwide in 2019 alone.26

As in adults, bacteria, viruses, and fungi cause pneumonia in children. But the specific bacteria, viruses, and fungi that cause pneumonia in children differ slightly from those that cause pneumonia in adults.6,7,26

  • Streptococcus pneumoniae: These bacteria are the most common bacterial cause of pneumonia among children. It’s widespread because the bacteria colonize the noses and throats of anywhere between 40%-50% of healthy children. It tends to be more common in children under the age of 2.26,27
  • Haemophilus influenzae type b (Hib): Hib is the second most common bacterial cause of pneumonia among children. These bacteria often pose no threat to human health, living in the noses or throats of healthy children. However, when they spread to other areas, such as the lungs, they can pose health risks.26,28
  • Mycoplasma pneumoniae: These bacteria are common among school-aged children and young adults, spreading via airborne droplets from coughs or sneezes. Short exposure to Mycoplasma pneumoniae often does not lead to infection. However, prolonged exposure increases the odds of transmission, which is why it tends to spread in schools.29,30
  • Respiratory syncytial virus (RSV): RSV is the most common viral cause of pneumonia among children. Symptoms of RSV infection often take the form of a mild cold, but the virus poses a serious health risk for young children. For example, RSV is the leading cause of pneumonia among children under 1 year old.17,18,26
  • Pneumocystis jirovecii: This fungus causes the most cases of pneumonia among children with compromised immune systems. They may be taking medications for HIV/AIDS or medications such as corticosteroids, which suppress the immune system. The fungus spreads in the air from person to person. Many people are exposed to Pneumocystis jirovecii, but their healthy immune systems typically prevent them from becoming sick.6,19,26
  • Children often carry the microbes that cause pneumonia in their noses and throats. These pathogens can lead to an infection when they reach the lungs. A person with an active infection may cough or sneeze, making the virus or bacteria airborne. Children then inhale the virus or bacteria, which may produce an infection in the lungs. The viruses and bacteria that cause pneumonia may also be spread to children during or after birth via blood.26

  • When a child under the age of 5 is coughing, healthcare providers may look for rapid breathing or a specific breathing pattern involving the retraction of the lower chest to identify a case of pneumonia in a child.26

    Other common pneumonia symptoms among children may include:

    • Cough26
    • Difficulty breathing26
    • Fever26
    • Vomiting or nausea31
    • Wheezing, which is more typical with viral pneumonia26
    • Loss of consciousness, convulsions, and/or hypothermia in severely ill infants26
  • Children may face increased risk of pneumonia when they have weakened immune systems, pre-existing conditions such as HIV/AIDS or measles, and environmental risk factors such as secondhand smoke, a crowded home, and indoor air pollution.26

  • Good nutritional health and steps to reduce indoor air pollution, as well as handwashing, can also help prevent pneumonia in children. Certain medications may help prevent pneumonia in children with HIV/AIDS, according to the World Health Organization.26

Pneumonia Symptoms

Pneumonia symptoms can vary from person to person, ranging from mild to severe. Some severe cases require hospitalization. Signs and symptoms of pneumonia may also vary based on the source of the infection, the patient’s health status, and their age.31 

General pneumonia symptoms may include:31 

  • Cough with mucus that can be yellow, green, or bloody
  • Fever, chills, and sweating
  • Chest pain that worsens with coughs or deep breaths and may produce a stabbing sensation
  • Appetite loss
  • Feeling tired
  • Nausea and vomiting (particularly in small children)
  • Confusion, which is more common among older people

Bacterial pneumonia tends to be more severe than other types of pneumonia, but it can be quick or slow to appear. Fevers may climb as high as 105 degrees and may produce profuse sweating. High pulse and breathing rates, lips and nailbeds that turn blue from the dip in oxygen levels, and delirium or confusion sometimes accompany bacterial pneumonia.31

Viral pneumonia symptoms typically emerge over the course of a few days and could be confused with symptoms of influenza. They can include weakness, fever, muscle pain, dry cough, and headache. The symptoms typically progress over a day or two, and cough may worsen along with muscle pain and breathing difficulty. People with viral pneumonia also may have blue lips.31 

In some cases, older adults and people who are immunocompromised may have fewer or more mild symptoms. They may even have a temperature that’s below normal. These people may also experience sudden shifts in mental clarity.31 

Fungal pneumonia symptoms can also differ. For example, coccidioidomycosis (valley fever) symptoms can include a rash, achy muscles and joints, and night sweats in addition to some general pneumonia symptoms.20,32

Pneumonia Complications

Certain people face increased risks of pneumonia complications. They include those with weakened immune systems or chronic health conditions, such as diabetes, as well as older adults and young children.33 Some complications may include: 

  • Bacteremia, a condition that may lead to septic shock after bacteria from an infection in the lungs circulate in the bloodstream. Septic shock is a serious health condition in which bacteria (and sometimes fungi or viruses) overload the body with toxins. This can cause organs to shut down and blood pressure to drop.33,34,35
  • Pockets of pus in the lungs called abscesses. Abscesses can lead to the death of lung tissue. Abscesses do not occur frequently, but they may require surgery to resolve.33,34,36
  • Pleural disorders, which affect the tissue surrounding the outside of the lungs and covering the interior of the chest. A pleural disorder can arise when an infection like pneumonia causes air or blood to accumulate in the pleural space, making breathing difficult.34,37
  • Renal failure, or failure of the kidneys. The kidneys help remove extra fluids, wastes, and minerals from the body. When they fail, these substances can accumulate, leading to serious complications.34,38
  • Respiratory failure, which occurs when the lungs fail to transfer sufficient oxygen to the blood.34,39

Diagnosis and Treatment

Pneumonia Diagnosis

Sometimes pneumonia symptoms mirror those of other conditions, such as the common cold, making it tricky to diagnose. To determine whether you have pneumonia, a healthcare provider may ask questions about your symptoms, gather a general health history, and listen to your lungs using a stethoscope.34 

Your healthcare provider may also order tests, such as a blood culture to determine the presence of bacteria in the blood, or lab tests, such as a complete blood cell count, which may help determine whether the immune system has been activated. An X-ray of the chest may also help identify pneumonia.34

Additional tests may include:34

  • Pulse oximetry/blood oxygen levels: Tests to establish the level of oxygen in the blood
  • Chest CT scan: An imaging test to see the extent of issues with the lungs
  • Sputum analysis: A test to identify the presence of bacteria in spit or phlegm
  • Bronchoscopy: A visual inspection of the airways
  • Pleural fluid culture: A test to detect bacteria in the fluid surrounding the lungs


Pneumonia Treatment

Healthcare providers select pneumonia treatments based on the source (bacterial, viral, or fungal) and severity of the infections. Healthcare providers may prescribe:40 

  • Antibiotics for bacterial pneumonia (but antibiotics are ineffective against viral pneumonia)
  • Antifungal medications for fungal pneumonia
  • Antiviral medicines for viral pneumonia, occasionally
  • Oxygen therapy for hospitalized patients

Healthcare professionals may also recommend several self-care options, including:40,41

  • Over-the-counter medications for muscle pain, fever, and trouble breathing
  • Rest
  • Hydration
  • Quitting smoking and avoiding secondhand smoke
  • Using a humidifier
  • Taking a warm bath
  • Isolating until symptoms resolve 

Global Impact of Pneumonia

Pneumonia has a vast global impact. The World Health Organization (WHO) reported that pneumonia caused 14% of all deaths among children under 5 years old in 2019 globally. The same year, pneumonia killed about 740,000 children worldwide.26 

Community-acquired pneumonia (CAP) is responsible for an estimated 3 million deaths globally each year. In Europe, CAP causes an estimated 23,000 deaths every year.12

Lower-income countries may have higher mortality rates than higher-income countries. For example, the mortality rates in Cambodia, Senegal, Uganda, and the Central African Republic are 23%, 19%, 18%, and 16%, respectively.12

One in nine patients around the world who become hospitalized for community-acquired pneumonia will be admitted to an intensive care unit with septic shock, respiratory failure, or severe sepsis. Overall, lower respiratory tract infections, which include pneumonia, cause 6.1% of deaths globally. They are the primary type of infection leading to death.12

Frequently Asked Questions About Pneumonia

  • People with pneumonia may experience a spectrum of symptoms. Some may have few or mild symptoms, but others may experience symptoms so severe they require hospitalization. Symptoms may vary depending on the different bacteria, viruses, and fungi that cause the pneumonia infection. Further, a person’s age and health status may affect how symptoms emerge.31

    Generally, pneumonia symptoms include:31

    • A cough that produces mucus, which can be green, yellow, or bloody
    • Cough and deep breaths may produce chest pain, which some describe as a stabbing sensation
    • Chills, fever, and sweating
    • Loss of appetite
    • Fatigue
    • Nausea and vomiting among children
    • Confusion among older adults

    Bacterial pneumonia tends to be more severe and sudden than viral pneumonia symptoms. Fevers can reach as high as 105 degrees. People with bacterial pneumonia may also have high heart and respiratory rates. As blood oxygen levels decline, lips and nail beds can turn blue and confusion can set in.31

    Viral pneumonia tends to have a more gradual onset, producing fever, muscle pain, headache, weakness, and dry cough. These symptoms typically worsen over a few days. Viral pneumonia may also sometimes lead to blue lips.31

    Symptoms vary among older adults and people who are immunocompromised. They may have milder symptoms or fewer symptoms, along with temperatures that fall below normal range.31

  • Yes, some of the viruses, bacteria, and fungi that cause pneumonia can spread. In some cases, the bacteria and viruses can spread via respiratory droplets. In other situations, pneumonia spreads when a person touches a surface contaminated with bacteria or viruses and then touches their face or nose. It’s unclear how long a person with viral pneumonia may spread the virus. Generally, with bacterial pneumonia, people may no longer spread the bacteria after 48 hours of antibiotics or without fever. Typically, as fever fades, so does a person’s ability to spread pneumonia. Fungal pneumonia does not spread from person to person.6

  • Walking pneumonia is an informal term that applies to pneumonia that doesn't require hospital treatment or bed rest. Despite this, walking pneumonia can still be serious. Symptoms can include headache, cough, chills, and chest pain. Over-the-counter treatments may provide relief. People may begin to feel better after three to five days, but cough can linger.11

  • Pneumonia symptoms vary by the individual and the cause. Some people have a few mild symptoms, others have many severe symptoms, while others fall somewhere in between. Some people with pneumonia can recover at home, while others need to be hospitalized. Age and how healthy a person is can also affect symptoms and symptom severity.31

    Pneumonia typically features the following symptoms:31

    • A cough with mucus that’s yellow, green, or bloody.
    • Fever, chills, and sweating
    • Chest pain from coughing, which some describe as a stabbing sensation
    • Appetite loss
    • Fatigue
    • Confusion in older adults
    • Nausea and vomiting in children

    Compared with viral pneumonia symptoms, bacterial pneumonia symptoms may progress faster and be more intense. For example, people can have 105-degree fevers, high respiratory and heart rates, and declines in blood-oxygen levels that leave lips and nail beds blue. They may also become confused.31

    Viral pneumonia tends to progress gradually, like the flu. Symptoms can include a dry cough, headache, weakness, fever, and muscle pain, all of which may worsen over a few days. Lips may also turn blue with viral pneumonia.31

    Older adults and people who are immunocompromised may have different symptoms. They may be milder, or these groups could have fewer symptoms. Their temperatures may also be below normal.31

  • Pneumonia is a lung infection affecting people of all ages, with a range of symptoms. Some people may have mild cases, others may have severe cases. Bacteria, viruses, and fungi can all lead to pneumonia.2,6

    Typically, people with pneumonia have a cough that may produce mucus. They may also have a fever accompanied by chills or sweating. Pneumonia may diminish their appetite and lower their overall energy levels. Because pneumonia affects the lungs, people with an active case may also experience shortness of breath or a stabbing sensation in the chest, along with general difficulty breathing. Older adults may appear to be confused, and children may have nausea and vomiting.31

    Some of the viruses, bacteria, and fungi that cause pneumonia can spread, while others cannot.6,31 In either case, pneumonia is a potentially serious lung disease. In severe cases, it can lead to hospitalization or be life-threatening.3,34

Find a Pfizer clinical trial for pneumonia at

Explore pneumonia clinical trials at

Area of Focus: Vaccines

Pneumonia is a focus of Pfizer’s Vaccines Therapeutic Area. Visit the Vaccines Page.

The information contained on this page is provided for your general information only. It is not intended as a substitute for seeking medical advice from a healthcare provider. Pfizer is not in the business of providing medical advice and does not engage in the practice of medicine. Pfizer under no circumstances recommends particular treatments for specific individuals and in all cases recommends consulting a physician or healthcare center before pursuing any course of treatment.