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Most of the time your body filters germs out of the air that you breathe. Sometimes germs, such as bacteria, viruses, or fungi, get into your lungs and cause infections.
When these germs get into your lungs, your immune system, which is your body's natural defense against germs, goes into action. Immune cells attack the germs and may cause inflammation of your air sacs, or alveoli. Inflammation can cause your air sacs to fill up with fluid and pus and cause the symptoms of pneumonia. Watch the video below to learn more. You can also learn more about How the Lungs Work in our Health Topic.
This video shows how your immune system tries to get rid of germs in your respiratory system and how you develop pneumonia. Medical Animation Copyright © 2020 Nucleus Medical Media Inc. All rights reserved.
Bacteria are a common cause of pneumonia in adults. Many types of bacteria can cause pneumonia, but Streptococcus pneumoniae (also called pneumococcus bacteria) is the most common cause in the United States.
Some bacteria cause pneumonia with different symptoms or other characteristics than “typical” pneumonia. This infection is called atypical pneumonia. For example, Mycoplasma pneumoniae causes a mild form of pneumonia often called “walking pneumonia.” Legionella pneumophila causes a severe type of pneumonia called Legionnaires’ disease. Bacterial pneumonia can happen on its own or develop after you have had a cold or the flu.
Viruses that infect your lungs and airways can cause pneumonia. The flu (influenza virus) and the common cold (rhinovirus) are usually the most common causes of viral pneumonia in adults. Respiratory syncytial virus (RSV) is the most common cause of viral pneumonia in young children.
Many other viruses can cause pneumonia, including SARS-CoV-2, the virus responsible for COVID-19. Watch our video on how SARS-CoV-2 affects the lungs to learn more. Additionally, we offer information and resources on how we are working hard to support necessary COVID-19 research.
Fungi such as Pneumocystis jirovecii may cause pneumonia, particularly in people who have weakened immune systems. Some fungi found in the soil in the southwestern United States and in the Ohio and Mississippi River valleys can cause pneumonia.