Causes and Risk Factors of Pneumonia

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Pneumonia is an infection in your lungs that inflames the air sacs, sometimes filling them with fluid or pus. It can be caused by a number of different organisms, including bacteria, viruses, and fungi, as well as by aspirating (inhaling) a foreign object. Pneumonia affects millions of people every year, particularly children under age 5 and adults over age 65, as well as people who are hospitalized or who have chronic illnesses or immunosuppression. Symptoms can range from mild to severe and include a productive cough, fever, chills, and difficulty breathing.

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Common Causes

Pneumonia isn't one single disease, and the cause determines what treatment is needed.


Bacterial pneumonia is the most common type of pneumonia and often only affects one area of your lung. It sometimes occurs after you've had some other type of infection that weakens your immune system, like a cold or the flu, but the condition can also occur on its own without a preceding infection.

Bacterial pneumonia is usually treated with antibiotics.

Examples of bacteria that can cause bacterial pneumonia include:

  • Streptococcus pneumoniae: This is by far the most common type of bacteria responsible for community-acquired bacterial pneumonia in the United States.
  • Haemophilus influenzae: These bacteria often cause pneumonia in older adults and people with pulmonary diseases such as cystic fibrosis and chronic obstructive pulmonary disease (COPD).
  • Mycobacterium tuberculosis: This is a common cause of pneumonia in people in developing countries, as well as in some parts of the United States.
  • Gram-negative bacilli: This group of bacteria doesn't typically cause pneumonia in the general population, but it's the second most common type associated with pneumonia that's severe enough to require hospitalization in the intensive care unit (ICU), after Streptococcus pneumoniae. Examples of gram-negative bacilli include Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter, Escherichia coli, Enterobacter, Serratia, and Proteus.
  • Anaerobes: These bacteria are associated with pneumonia caused by aspirating (inhaling) food, drink, saliva, or vomit into your lungs.

Atypical Bacteria

Atypical bacteria are bacteria that are unable to be cultured with standard methods. Atypical bacteria that cause pneumonia include:

  • ​​​Mycoplasma pneumoniae: This bacterium causes a type of pneumonia referred to as "walking pneumonia," which typically has mild symptoms, and responds to antibiotics. Because it's contagious, living or working in crowded places like dormitories, schools, or prisons increases your risk of picking up this type of pneumonia.
  • Chlamydia pneumoniae: This bacterium also usually causes an infection with mild symptoms and mostly affects adults ages 40 to 59.
  • Legionella pneumophila: Pneumonia related to this type of bacteria is also known as Legionnaire's disease. It is typically transmitted by inhaling aerosols that contain it, and outbreaks have been connected with exposure to whirlpool spas, showers, fountains, and cooling towers.


Viral pneumonia, by definition, is caused by a virus and is the most common cause of pneumonia in children under age 5. It's usually not as serious as bacterial pneumonia, though it puts you at a higher risk of developing it.

Most people with viral pneumonia recover within one to three weeks without treatment, though some cases become severe and require hospitalization.

Examples of viruses that can cause viral pneumonia include:

  • Influenza viruses: Influenza A, B, and avian flu viruses can cause pneumonia, particularly in adults. 
  • Respiratory syncytial virus (RSV): RSV is the most common cause of viral pneumonia in infants under a year old, but it can also cause pneumonia in any age group and can be especially severe in adults over 65 and in people whose immune systems are compromised.
  • Human parainfluenza viruses: These viruses cause respiratory infections, including pneumonia, in people of all ages, especially in young children, the elderly, and people with suppressed immune systems.
  • Adenovirus: This type of virus can cause anything from a cold to a sore throat, bronchitis to pneumonia.
  • Rhinovirus: This is the virus that causes the common cold, which can lead to pneumonia.
  • Human metapneumovirus (HMPV): This is yet another respiratory virus that can cause pneumonia, particularly in young children and older adults.


Fungal pneumonia is caused when spores enter your lungs and multiply. It commonly occurs in people with compromised immune systems or chronic health problems. However, it can occur in otherwise healthy adults too.

Fungal infections that are picked up from the soil in certain areas of the United States can lead to fungal pneumonia, including:

  • Pneumocystis pneumonia: This infection, caused by the Pneumocystis jirovecii fungus, often causes serious pneumonia and usually occurs in people whose immune systems are suppressed, such as those with human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome (AIDS), people who are undergoing cancer treatments, and those who have had organ transplants.
  • Coccidioidomycosis: Also known as "valley fever," the fungus that causes this infection, Coccidioides, is found in southern Arizona, central California, southwestern New Mexico, and west Texas.
  • Histoplasmosis: The Histoplasma capsulatum fungus is found in the Ohio and Mississippi River valleys and is spread through soil that's contaminated by bird and bat droppings. Extensive exposure to highly contaminated areas causes people to develop pneumonia.
  • Cryptococcus: The Cryptococcus fungus is found in soil all over the world, but pneumonia typically only occurs in people with compromised immune systems.


Aspiration happens when a foreign object, such as a piece of food, gum, liquid, or vomit, is inhaled. The object then becomes lodged in one or both lungs where it's trapped, unless you're able to cough it up. When you aspirate a foreign object or liquid, it increases the presence of bacteria as the object begins to rot, leading to an infection. 

Aspiration pneumonia, or the collection of infectious material in the lungs due to the presence of a foreign object, can make it difficult to breathe.

Some people may have an ongoing condition where they accidentally swallow food into their lungs instead of into their esophagus, the tube that moves food to the stomach. Many people never even realize that a piece of food has gone down the "wrong pipe." This problem is more common in the elderly, who are more likely to have problems with swallowing. The risk of pneumonia due to aspiration while under anesthesia is also why patients are asked not to eat or drink prior to surgery.

Health Risk Factors

Pneumonia can affect anyone at any age, but the two age groups at the highest risk both for contracting it and for having more severe cases are children under age 2 and adults over age 65.

Other risk factors include:

  • Being in the hospital: Because your immune system is already weakened, your risk of developing pneumonia is higher if you're hospitalized in the ICU. Your risk is even higher if you're on a ventilator to help you breathe.
  • Having a chronic disease: If you have COPD, asthma, heart disease, bronchiectasis, cystic fibrosis, diabetes, celiac disease, or sickle cell disease, your risk of contracting pneumonia is higher than that of the general population.
  • Having a suppressed immune system: If you have HIV or AIDS, have had an organ or bone marrow transplant, are receiving chemotherapy or long-term steroids, or have an autoimmune disorder, you're at higher risk for pneumonia.
  • Difficulty swallowing: If you have a hard time swallowing due to a condition like Parkinson's disease or because of a stroke, you're at a higher risk of aspirating food, drink, saliva, or vomit and, thus, developing aspiration pneumonia.
  • Reduced consciousness: Whether you're sedated, prone to generalized seizures, or have had anesthesia, these episodes of reduced consciousness can contribute to aspiration pneumonia.
  • Difficulty coughing: Not being able to cough properly or often enough can lead to pneumonia.

Lifestyle Risk Factors

Risk factors that may have to do with your lifestyle choices increase your likelihood of developing pneumonia and include:

  • Smoking: If you smoke, your risk of contracting pneumonia is higher than that of the general population because smoking compromises your immune system's ability to defend itself from the organisms that make you sick.
  • Drug or excessive alcohol use: Drinking too much alcohol or using drugs is another risk factor for pneumonia because you may aspirate food, drink, or vomit into your lungs while you're under the influence.
  • Malnutrition: Being undernourished contributes to a higher risk of developing pneumonia and of it being more severe, especially in young children and older adults. It's estimated that malnutrition is the underlying cause of death in 45 percent of children under the age of 5 worldwide.
  • Poor dental health: Poor oral hygiene can contribute to pneumonia, especially if you have dentures.
  • Exposure to animals, chemicals, or environmental toxins: Being around animals can expose you to infected droppings that get in the soil. Certain chemicals and pollutants may also increase your risk of pneumonia.

It's important to be aware that your risk of pneumonia increases with each additional health or lifestyle risk factor you have.

Frequently Asked Questions

  • Does age affect whether you’re at risk for pneumonia?

    Yes. Children under 2 years of age are at risk because their immune systems are still developing, and adults over age 65 have an increased risk because, as we age, our immune systems don't fight off infections as well.

  • Which types of pneumonia are contagious?

    Bacterial pneumonia is contagious, with infections caused by Mycoplasma pneumoniae (walking pneumonia) and Mycobacterium tuberculosis being among the most highly contagious. Viral pneumonia is also contagious. Fungal and aspiration pneumonia are not transmitted from person to person.

  • Why is pneumonia caused by COVID-19 so serious?

    When the COVID-19 virus causes pneumonia, it typically involves both lungs. Usually, people recover from other types of pneumonia without lasting lung problems, but COVID-19-related pneumonia often leaves long-term scarring and stiffness that leads to lingering interstitial lung disease in which it’s difficult to get enough air into your body.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.