Pneumonia is a lower respiratory tract infection of the lungs. It can affect one or both lungs. As the infection takes hold, it causes the respiratory air sacs in the lungs to fill with secretions and affects your ability to breathe.
Bacteria, viruses, or fungi can cause this infection. The underlying cause, along with your age, health, and other factors, determines how severe your pneumonia might become.
Pneumonia is often divided into types depending on how it is contracted:
Pneumonia is considered a community-acquired infection when contracted outside a healthcare setting. This includes cases discovered within 24 hours of checking into a hospital, as they were probably acquired outside.
Pneumonia that develops more than 48 hours after a patient is admitted to the hospital is usually called hospital-acquired pneumonia (HAP).
However, if the condition is related to the use of a mechanical ventilator, it’s called ventilator-associated pneumonia (VAP).
The third subtype is healthcare-associated pneumonia (HCAP). This applies to patients that aren’t hospitalized but have had significant recent contact with the healthcare system.
When you get sick with a viral infection (like the common cold or flu), your body’s immune system tries to fight it off. You could also become infected with bacteria while your immune system is weakened from the first infection.
This type of secondary bacterial infection is a significant cause of severe pneumonia, particularly in those with the flu or COVID-19.
Globally, pneumonia is a leading cause of death for children under five. Just a few decades ago, it caused the deaths of more children under five years than people aged 70 or over.¹ That number has declined drastically since then.
Some reasons for the decline in severe pneumonia cases in young children are the availability of antibiotics, the pneumococcal vaccine (which helps prevent infection from a bacteria that frequently causes pneumonia), better air quality, and improved nutrition.²
Each year, around 1.5 million people in the United States visit an emergency department with a primary diagnosis of pneumonia.³ Lower respiratory tract infection is the most common infectious cause of death worldwide. The infection causes close to 50,000 deaths per year.³
Children aged two years and under and adults over the age of 65 are most at risk of developing pneumonia.⁴ Environmental and other factors may also increase your chances of developing the infection.
Death from pneumonia is strongly linked to poverty-related conditions, air pollution, and lack of adequate medical care.⁵
Death rate from pneumonia, by age, World, 1990 to 2019 | Our World in Data
Pneumonia | Our World in Data
Pneumonia | Centers for Disease Control and Prevention (CDC)
Pneumonia: Causes and risk factors | NIH: National Heart, Lung, and Blood Institute
Pneumonia | Unicef
The severity of pneumonia symptoms varies depending on the cause and how well your immune system responds to the infection.
Most people with pneumonia experience one or more of the following signs or symptoms:¹
Fever and chills
Pain in the chest when breathing or coughing
Shortness of breath and difficulty breathing
In addition to pus forming in the lungs (in bacterial pneumonia), the infection can cause other complications, including:²
Fluid around the lungs. The area surrounding the lungs is called the pleural space. A condition called pleural effusion occurs when this area fills with fluid.³
Bacteremia. A condition called bacteremia occurs when bacteria enter your bloodstream. This can lead to a much more severe systemic form of the illness, such as sepsis.
Sepsis. With pneumonia, sepsis occurs when the body’s immune response to the infection causes inflammation. This inflammation results in clots or leaking blood vessels and can deprive organs of the oxygen they need to function, causing them to shut down. This eventually leads to organ failure.
Pneumonia: Symptoms | NIH: National Heart, Lung, and Blood Institute
Typical bacterial pneumonia | NIH: National Library of Medicine
Pleural effusion: diagnosis, treatment, and management (2012)
Bacteria, viruses, and fungi can cause infections that lead to pneumonia. Determining which organism is causing the underlying infection is a vital part of developing an appropriate treatment plan.
Many types of bacteria can infect the lung, but the most common cause of bacterial pneumonia in the US is Streptococcus pneumoniae.¹
Some bacterial infections present with symptoms that are different from a typical case of pneumonia. These are known as atypical-type pneumonias.
Types of bacteria that can cause pneumonia include:
Streptococcus pneumoniae. This organism causes pneumococcal pneumonia. It is also responsible for infections in other parts of the body, including the blood, sinuses, middle ear, and brain. It spreads through direct contact with saliva or mucus.
Legionella pneumophila. This bacteria causes more severe pneumonia called Legionnaires’ disease. Instead of spreading from person to person, it is usually contracted by breathing the mist of water contaminated with Legionella.²
Bordetella pertussis. Whooping cough, caused by Bordetella pertussis, can lead to pneumonia. The infection is extremely contagious and spreads through coughing, sneezing, and sharing breathing space for an extended time.
Mycoplasma pneumoniae and Chlamydia pneumoniae. These bacteria cause atypical-type pneumonia. Usually manifesting in a milder form, these infections are often referred to as walking pneumonia.
Any virus that infects the lungs and upper respiratory system can cause pneumonia.
Up until the pandemic, viral pneumonia in adults was most often caused by influenza and the common cold. These infections typically begin in the upper respiratory tract and move downward in some people to cause pneumonia. However, children are more likely to develop pneumonia from respiratory syncytial virus (RSV).
Viruses that can cause pneumonia include:
SARS-CoV-2.SARS-CoV-2 — the virus that causes COVID-19 — usually causes an upper respiratory infection. However, it can infect the lower respiratory tract in some cases, leading to pneumonia. It spreads when infected people breathe out tiny droplets containing the virus encountered or inhaled by someone else. Because of how SARS-CoV-2 affects certain cell types in the lungs, these cells can become damaged and die.³ The lung surface tension then decreases, which affects the air sacs responsible for respiration. Pneumonia can develop when the air sacs collapse.
Human metapneumovirus (HMPV). A seasonal virus that spreads from winter through spring, HMPV is also known to cause upper and lower respiratory disease.⁴ Like most other viral respiratory infections, it’s spread through respiratory secretions. Infections from HMPV are more common in children but can also be serious in some adults with preexisting conditions, like heart disease, a lowered immune system, chronic lung disease, and blood disorders.
Human parainfluenza virus (HPIV). HPIV typically causes upper respiratory tract infections. However, it can also move lower to cause bronchitis and pneumonia, particularly in more vulnerable groups, such as young children and older adults. This seasonal virus spreads through coughing, sneezing, and contact with respiratory secretions. Infections are most common in spring, summer, and fall.⁵
Influenza (flu). The influenza virus is present at low levels year-round, but people typically catch the flu when it peaks during the fall and winter flu seasons. Influenza virus is a common cause of viral pneumonia and is spread by air droplets from up to six feet away. Older people, those who are immunocompromised, and people with conditions such as chronic lung disease are more susceptible to developing pneumonia from having the flu. Around one-third of people who are hospitalized for serious influenza infections also have pneumonia.⁶
Rhinovirus. Of the viruses that cause the common cold, this is one of the most prevalent.⁷ Rhinovirus can spread through the air and through contact with respiratory secretions.
Respiratory syncytial virus. RSV is another virus that often causes cold symptoms. Like most other respiratory viruses, it spreads through air droplets and contact with respiratory secretions. Children, premature infants, older adults, and other high-risk individuals are more likely to develop pneumonia as the virus can travel to the lower respiratory tract more easily.⁸
Certain types of fungus can also cause pneumonia. These include:
Pneumocystis jirovecii. Getting sick from Pneumocystis jirovecii is extremely rare. In fact, 20% of people may carry this fungus in their lungs at any time.⁹ The immune system usually removes it after a few months without it ever having caused symptoms. But those with weakened immunity might not be able to fight off the infection and may go on to develop pneumonia. The fungus spreads through the air.
Coccidioidomycosis. This infection is transmitted from contact with dust particles carrying spores of the Coccidioides fungus. Certain geographical locations, including Texas and Arizona, have this fungus in the soil. People can breathe in the spores and become infected when the soil is disturbed. Some cases progress to pneumonia, particularly in people who are immunocompromised.
Aspergillus. Aspergillus can cause complications like allergic bronchopulmonary aspergillosis (ABPA) in people who are immunocompromised or have asthma. ABPA can lead to pneumonia.¹⁰
Some factors make you more susceptible to developing a pneumonia infection, including:
Age. Infants and young children aged two years and under and people aged 65 or above have a higher risk of developing pneumonia. This increased risk is due to the developing immune system of young children and the likelihood of additional health problems and weakening immunity in older people.
Environment. Factors that negatively impact your respiratory tract can play a role in increasing your risk of pneumonia. These include job site hazards, such as harsh chemicals, smoke, and other irritants, as well as environmental factors, like heavy smog.¹¹
Smoking. Smoking cigarettes makes it more difficult for you to clear mucus from your airways, providing a more fertile ground for infection. Vaping has also been shown to contribute to lung damage.¹²
Lung disease. Long-term lung conditions, such as asthma, bronchiectasis, cystic fibrosis, and chronic obstructive pulmonary disease (COPD), also cause damage to lung tissue and increase the chances of infection.
Drugs or alcohol. Drinking alcohol and using other recreational drugs harms your body and weakens your immune system. This makes it easier for the microbes responsible for pneumonia to take hold and harder for your body to fight them off.
Brain disorders. Certain central nervous system conditions can impact your ability to cough or swallow. A brain condition that causes aspiration (when food, drink, or saliva goes down your windpipe instead of your esophagus) will increase your likelihood of contracting aspiration pneumonia. Clearing lung secretions when you can’t cough is also difficult, so even common respiratory infections can lead to you developing other types of pneumonia.
Critical disease. Hospital-acquired pneumonia is more likely in people with severe, life-threatening medical conditions (like heart disease, diabetes, stroke, or cancer) as they are more likely to spend time in the hospital. These diseases and their treatments may also weaken the immune system, further increasing the risk of pneumonia.
Evolving understanding of the causes of pneumonia in adults, with special attention to the role of pneumococcus (2017)
Human metapneumovirus as cause of severe community-acquired pneumonia in adults: insights from a ten-year molecular and epidemiological analysis (2019)
Transmission: How HPIVs spread | Centers for Disease Control and Prevention (CDC)
Common colds: Protect yourself and others | Centers for Disease Control and Prevention (CDC)
Host components contributing to respiratory syncytial virus pathogenesis (2019)
Pneumocystis pneumonia | Centers for Disease Control and Prevention (CDC)
Pulmonary aspergillosis: clinical presentation, diagnostic tests, management and complications (2012)
Advances in the prevention, management, and treatment of community-acquired pneumonia (2016)
E-cigarette, or vaping, product use-associated lung injury: a review (2020)
Getting a pneumonia diagnosis puts you on the path to treating the condition.
Make an appointment with your doctor if you develop pneumonia symptoms.
Your doctor will typically check your vital signs to see if you are running a fever. Then they will assess your oxygen status.
Next, they will take your medical history. They will ask about your medications, recent travel, and recent contact with people who are unwell. This will help your doctor understand your risk factors or what you might have been exposed to recently.
Your doctor will then conduct a physical exam, checking for signs of infection and listening to your chest for abnormal lung sounds, respiratory distress, or congestion.
Further tests can help verify your diagnosis, including:
Chest x-ray. An x-ray of your lungs can indicate an infection, inflammation, or fluid.
Blood tests. Certain blood tests will show your doctor if your immune system is actively fighting off an infection.
Pulse oximetry. A pulse oximeter device can be attached to your finger to measure the percentage of oxygen in your blood using light. Because pneumonia restricts airflow and oxygen exchange in the lungs, the amount of oxygen in your blood will sometimes be lower than normal. This depends on the severity of the infection. This test is usually done while taking your vital signs.
The doctor may perform additional tests if you have other health conditions or have recently been in hospital due to another medical condition.
The first line of defense when treating pneumonia is to address the underlying cause. In more severe cases, your doctor may choose to hospitalize you to provide oxygen, monitor your status, and give IV antibiotics to help fight the infection. This is usually the procedure when your oxygen levels are affected.
Below is a summary of the different types of pneumonia treatment:
Medication. You will be treated with antibiotics, antiviral drugs, or antifungal drugs. The medication you take will depend on the type of pneumonia. Your doctor may recommend other medications to help control your symptoms.
Oxygen. This is a key treatment for low oxygen levels and respiratory distress. Oxygen therapy can help you through the worst period of the illness. If the need for oxygen is ongoing, you will likely need to be hospitalized.
Hospitalization. Doctors in hospitals can start an IV to ensure you’re getting the right amount of fluids, monitor your respiratory status, and keep you properly oxygenated.
Home remedies. Staying hydrated is important when you have pneumonia, so be sure to drink plenty of fluids. Try to stay warm and get plenty of rest to help your immune system fight off the illness.
Pneumonia is caused by pathogens you pick up from the air or by close personal contact. The best way to prevent pneumonia is to protect yourself from exposure and bolster your immune system for when you are exposed. You can do this by:
Immunization. Vaccinations are available for SARS-CoV-2, influenza, and several other microbes that can cause or result in pneumonia. To minimize your risks, ask your doctor which vaccinations you should get and when they are needed.
Nutrition, rest, and exercise. A healthy immune system is a key to fighting the infections that lead to pneumonia before it develops. Eating a nutritious diet, doing enough exercise, and getting enough rest will help keep your immune system healthy.
Wash your hands regularly. Many of the pathogens that can lead to pneumonia are spread through contact with respiratory secretions. Washing your hands fully and regularly will help you avoid becoming infected or spreading pathogens to others.
To keep your risk of pneumonia as low as possible, you can try to minimize your exposure to the known risk factors and live a healthy, balanced lifestyle. Risk factors in your control include smoking, using drugs and alcohol, and exposure to harmful environmental factors.
Lower respiratory infections can be deadly if left uncontrolled. Contact your doctor as soon as possible if you think you’ve developed pneumonia.
Seek urgent medical care if you have difficulty breathing or chest pain as you breathe, especially if you are in a high-risk group.
Your primary care physician can treat and help you recover from pneumonia in most cases. However, severe infections might require the assistance of a pulmonologist — particularly with underlying lung conditions. A pulmonologist is a doctor that specializes in respiratory tract conditions.