What Can Cause Pneumonia?

Experts explain what's most likely to cause pneumonia—and ways you can avoid getting the lung infection.

The main symptoms of pneumonia—trouble breathing, chest pain, cough, and fever—are usually the same, no matter what type of pneumonia you have. But there's more than one cause of pneumonia, and figuring out what's behind your illness can help a healthcare provider effectively treat your condition.

Pneumonia is an infection of the lungs that causes inflammation and fluid buildup in the alveoli, which are tiny air sacs in your lungs. Pneumonia can cause mild to severe illness in people of all ages. Still, you're more likely to get pneumonia if you smoke or have underlying health conditions like diabetes or heart disease.

You can help protect against pneumonia. Some vaccines may protect against and lessen symptoms of some types of pneumonia. For example, being vaccinated against the flu can less your risk of being sick with the flu. Therefore, your risk of pneumonia as a complication of the flu also lowers. Also, practicing good hand hygiene and disinfecting frequently touched surfaces are helpful.

According to data from the Centers for Disease Control and Prevention (CDC), more than 47,000 people died from pneumonia in the United States in 2020.

Arming yourself with all the facts about pneumonia—including what most commonly causes the lung infection—can help you stay as pneumonia-free as possible.

Here, experts explained what you need to know about the three main causes of pneumonia.


Most commonly, the bacterium Streptococcus pneumoniae infects and colonizes in the upper respiratory tract in cases of bacterial pneumonia.

Bacterial pneumonia causes symptoms such as:

  • Fever and chills
  • Sweating
  • Coughing
  • Rapid breathing or difficulty breathing
  • Chest pain

Bacterial pneumonia may develop as a complication of a viral infection, like the cold or flu. Or the illness can come about due to aspirating or inhaling fluid like saliva or vomit. 

And in some situations, a viral infection that disturbs the balance of bacteria in your lungs can cause bacterial pneumonia, Marc A. Sala, MD, a pulmonologist and assistant professor of medicine at Northwestern Medicine, told Health

"It used to be thought that the lungs were a sterile environment, devoid of bacteria. We now know that this is not true and that the lungs are 'colonized' by bacteria that are harmless under ordinary circumstances," said Dr. Sala.

In addition to Streptococcus pneumoniae, other bacteria known to cause bacterial pneumonia include:

  • Staphylococcus aureus
  • Group A Streptococcus
  • Klebsiella pneumoniae
  • Haemophilus influenzae
  • Moraxella catarrhalis

Also, some bacteria cause what's known as "atypical pneumonia." Those bacteria, which are usually difficult to detect using standard methods, include:

  • Legionella pneumophila (pneumonia caused by this bacteria is more commonly known as Legionnaires' disease)
  • Mycoplasma pneumoniae
  • Chlamydia pneumoniae
  • Chlamydia psittaci

Usually, a healthcare provider will treat bacterial pneumonia with oral antibiotics. In severe cases, treatment includes broad-spectrum antibiotics, which target several types of bacteria.

The pneumococcal vaccine can help lower your risk of contracting bacterial pneumonia from Streptococcus pneumoniae. In the United States, two kinds of vaccines can help protect against the bacterium.


There are a lot of viruses that can lead to viral pneumonia. Respiratory syncytial virus (RSV) is one of the most common causes of viral pneumonia in children. In adults, the influenza virus and SARS-CoV-2, the virus that causes COVID-19, are among the main causes of pneumonia. Also, the common cold may lead to pneumonia.

Viral pneumonia causes symptoms that may include:

  • Cough
  • Fever
  • Chills
  • Shortness of breath
  • Confusion (especially in older adults)
  • Sweating
  • Headache
  • Loss of appetite
  • Chest pain
  • Fatigue

"The typical way that one develops pneumonia is by being exposed to the droplets or aerosols of someone who has an active infection, resulting in inflammation and an immune response, which we then refer to as pneumonia," explained Dr. Sala. 

With mild cases, managing your symptoms and resting can help treat viral pneumonia. Other viral pneumonia treatments usually involve using an antiviral treatment like Tamiflu (oseltamivir) or remdesivir. And if your symptoms do not improve with treatment or become severe, it's essential to consult a healthcare provider.

Some vaccines that help lower your risk of developing other viral infections, including the flu and COVID-19, may prevent viral pneumonia that develops as a complication of those illnesses.


Fungal pneumonia is more of a potential issue for people with weakened immune systems than others. For example, people who are human immunodeficiency virus (HIV)-positive and have progressed to acquired immune deficiency syndrome (AIDS) are at risk for fungal pneumonia.

Also, people exposed to large doses of certain fungi from contaminated soil or bird droppings may develop fungal pneumonia.

Also, people exposed to large doses of certain fungi from contaminated soil or bird droppings may develop fungal pneumonia. Still, healthcare providers "don't really see fungal pneumonia in people with normal immune systems," Nicola Hanania, MD, a pulmonologist at the Baylor College of Medicine, told Health

"By and large, fungal pneumonia is not an issue for most people in the U.S., with the exception of a few organisms," added Dr. Sala.

The fungal infections most commonly linked to pneumonia include:

  • Pneumocystis pneumonia (PCP), caused by the fungus Pneumocystis jiroveci
  • Valley fever or coccidioidomycosis, caused by the Coccidioides fungus
  • Histoplasmosis, caused by the Histoplasma fungus
  • Cryptococcosis, caused by the fungi Cryptococcus neoformans and Cryptococcus gattii

With fungal pneumonia, people are often infected when they breathe in certain fungal spores. Healthcare providers will usually treat fungal pneumonia with antifungal medications like fluconazole.

A Quick Review

Pneumonia is an infection of the lungs that can range from mild to severe. Different types of microorganism cause pneumonia, including bacteria, viruses, and fungi. 

Treating your symptoms is essential if you're experiencing typical signs of pneumonia, fever, chills, cough, and shortness of breath. Some types of pneumonia require antibiotics or antifungal medicines. Generally, mild cases of viral pneumonia usually subside with rest and fluids.

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15 Sources
Health.com 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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