Pneumonia causes

Causes, risk factors and prevention

  • You can get pneumonia from bacterial, fungal, or viral infections.
  • You can also get aspiration pneumonia, which is when you inhale food, or hospital-acquired pneumonia.
  • To prevent pneumonia, get your flu shot, wash your hands and stop smoking.
  • Visit Insider’s Health Reference Library for more tips.

Pneumonia is a lung infection that can be caused by bacteria, fungi, or viruses. Symptoms of pneumonia include shortness of breath, fever, and cough.

Here are the top five causes of pneumonia and how you can prevent the disease.


Bacterial pneumonia is any pneumonia caused by bacteria and is usually treated with


. More than 900,000 Americans develop bacterial pneumonia each year.

The most common cause of bacterial pneumonia in the United States is Streptococcus pneumoniae, which is usually found in the nose and throat. The bacteria are spread by drops of saliva or mucus when people cough or sneeze. Although many of us carry this germ without getting sick, it often causes pneumonia in the elderly, young children, and smokers or those with weaker immune systems due to illness or medication. .

Although rarer than strep pneumonia, other bacteria can also cause the disease. This is called atypical pneumonia or gait pneumonia. Foot pneumonia often has milder symptoms and does not require bed rest.


The spores of some fungi, if inhaled in large doses, can also lead to pneumonia. In 2011, more than 20,000 cases of fungal pneumonia were reported in the United States, making this type of pneumonia less common.

Fungi that can cause pneumonia include:

  • Histoplasmosis, which is common in the Ohio and Mississippi valleys.
  • Cryptococcus, which is found in bird droppings nationwide.
  • Coccidioidomycosis, which lives in the soil of the Southwest.

Farmers, farm laborers, and landscapers are at a higher risk of developing fungal pneumonia because they work in close contact with the soil which can carry these fungi.

In addition, the pneumocystis jirovecii fungus can cause a serious condition called pneumocystis pneumonia (PCP) in people who are immunocompromised, such as people living with HIV or recent organ recipients. Fungal pneumonia can be treated with antifungal drugs.


Certain viruses, such as those that cause conjunctivitis or the flu, can also cause pneumonia. Viral pneumonia accounts for about one in three cases of pneumonia, although these cases tend to be milder and shorter than bacterial pneumonia because there is less inflammation.

Respiratory illnesses, such as the flu or COVID-19, can lead to pneumonia by causing inflammation and fluid to build up in the lungs, making it harder to breathe.

There is little or no treatment for most viruses that cause pneumonia, which means patients have to wait for the infection to pass. However, if doctors think an influenza virus is to blame, they may prescribe antiviral therapy or other medications to make breathing easier.

Nosocomial pneumonia

Nosocomial pneumonia refers to any pneumonia acquired 48 hours or more after admission to hospital. The disease is serious not only because patients’ immune systems can be weakened by other illnesses, but also because bacteria found in hospitals can be resistant to antibiotics.

The causes of nosocomial pneumonia include:

  • Healthcare workers can spread it with their hands, clothes or instruments as they pass from patient to patient.
  • Poor oral hygiene, which means bacteria in saliva are more likely to enter the lungs while breathing, speaking or eating, says Karen Giuliano, associate professor of innovation and product development medical at the University of Massachusetts. This is called microaspiration.

Microaspirations can become dangerous in a hospital setting because patients – and their mouths – not only come into contact with more bacteria, but with different types than we would have at home. Meanwhile, oral hygiene in inpatients tends to be a low priority, as healthcare workers focus on the issues for which patients have been admitted – increasing the levels of bacteria in the mouth, she says.

Making sure people have access to good oral hygiene, such as brushing their teeth, while in hospital can reduce the risk of hospital-acquired pneumonia.

Ventilator-associated pneumonia

This is another form of pneumonia that occurs when people using breathing apparatus contract pneumonia from germs entering their respiratory tract. It is classified as a distinct form of pneumonia distinct from nosocomial pneumonia, despite its hospital setting.

Aspiration pneumonia

Aspiration pneumonia is a type of pneumonia that occurs when food or liquid is inhaled into the lungs, causing infection. The phenomenon is one of the most common causes of pneumonia in patients in long-term care facilities.. Although cases of the disease have declined over the past decade or so, there remains a particular risk for patients over 65.

Who is at risk for pneumonia?

Some people are at greater risk of developing pneumonia. Groups at risk include:

  • People over 65
  • Children under two
  • People with weaker immune systems due to illness, such as diabetes and cancer, as well as those with chronic lung disease
  • Smokers
  • People taking drugs like chemotherapy and long-term steroids
  • Hospitalized patients, including those on ventilators
  • People living with HIV, especially those not yet on antiretroviral therapy or whose HIV is poorly controlled

How to prevent pneumonia

There are several things you can do to reduce your risk of pneumonia, including:

In addition, anyone admitted to the hospital should make sure to bring their own oral hygiene kit. Giuliano suggests packing:

  • A high quality soft bristle toothbrush
  • Toothpaste that contains substances such as fluoride to help break down plaque
  • Alcohol-free antiseptic mouthwash

Insider’s takeaway

You can get pneumonia caused by bacteria, viruses or fungi. Pneumonia can affect young and old, but people who are hospitalized or whose immune systems are weaker because of their age or underlying health problems are at greater risk.

Vaccinations, frequent hand washing, and chronic disease management can help lower your risk of pneumonia. Hospital patients should also make sure to practice good oral hygiene when in facilities and try to be as mobile as possible.