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Pneumonia is an infection in one or both of the lungs. It causes the air sacs of the lungs to fill up with fluid or pus. It can range from mild to severe, depending on the type of germ causing the infection, your age, and your overall health.

Bacterial, viral, and fungal infections can cause pneumonia.

Bacteria are the most common cause. Bacterial pneumonia can occur on its own. It can also develop after you've had certain viral infections such as a cold or the flu. Several different types of bacteria can cause pneumonia, including:

• Streptococcus pneumoniae
• Legionella pneumophila; this pneumonia is often called Legionnaires' disease
• Mycoplasma pneumoniae
• Chlamydia pneumoniae
• Haemophilus influenzae

Viruses that infect the respiratory tract may cause pneumonia. Viral pneumonia is often mild and goes away on its own within a few weeks. But sometimes it is serious enough that you need to get treatment in a hospital. If you have viral pneumonia, you are at risk of also getting bacterial pneumonia. The different viruses that can cause pneumonia include:

• Respiratory syncytial virus (RSV)
• Some common cold and flu viruses
• SARS-CoV-2, the virus that causes COVID-19

Fungal pneumonia is more common in people who have chronic health problems or weakened immune systems. Some of the types include:

• Pneumocystis pneumonia (PCP)
• Coccidioidomycosis, which causes valley fever
• Histoplasmosis
• Cryptococcus

Anyone can get pneumonia, but certain factors can increase your risk:

• Age; the risk is higher for children who are age 2 and under and adults age 65 and older
• Exposure to certain chemicals, pollutants, or toxic fumes
• Lifestyle habits, such as smoking, heavy alcohol use, and malnourishment
• Being in a hospital, especially if you are in the ICU. Being sedated and/or on a ventilator raises the risk even more.
• Having a lung disease
• Having a weakened immune system
• Have trouble coughing or swallowing, from a stroke or other condition
• Recently being sick with a cold or the flu

The symptoms of pneumonia can range from mild to severe and include:

• Fever
• Chills
• Cough, usually with phlegm (a slimy substance from deep in your lungs)
• Shortness of breath
• Chest pain when you breathe or cough
• Nausea and/or vomiting
• Diarrhea

The symptoms can vary for different groups. Newborns and infants may not show any signs of the infection. Others may vomit and have a fever and cough. They might seem sick, with no energy, or be restless.

Older adults and people who have serious illnesses or weak immune systems may have fewer and milder symptoms. They may even have a lower than normal temperature. Older adults who have pneumonia sometimes have sudden changes in mental awareness.

Sometimes pneumonia can cause serious complications such as:

• Bacteremia, which happens when the bacteria move into the bloodstream. It is serious and can lead to septic shock.
• Lung abscesses, which are collections of pus in cavities of the lungs
• Pleural disorders, which are conditions that affect the pleura. The pleura is the tissue that covers the outside of the lungs and lines the inside of your chest cavity.
• Kidney failure
• Respiratory failure

Sometimes pneumonia can be hard to diagnose. This is because it can cause some of the same symptoms as a cold or the flu. It may take time for you to realize that you have a more serious condition.

Your health care provider may use many tools to make a diagnosis:

• A medical history, which includes asking about your symptoms
• A physical exam, including listening to your lungs with a stethoscope
• Various tests, such as
  • A chest x-ray
  • Blood tests such as a complete blood count (CBC) to see if your immune system is actively fighting an infection
  • A Blood culture to find out whether you have a bacterial infection that has spread to your bloodstream

If you are in the hospital, have serious symptoms, are older, or have other health problems, you may also have more tests, such as:

• Sputum test, which checks for bacteria in a sample of your sputum (spit) or phlegm (slimy substance from deep in your lungs).
• Chest CT scan to see how much of your lungs is affected. It may also show if you have complications such as lung abscesses or pleural effusions.
• Pleural fluid culture, which checks for bacteria in a fluid sample that was taken from the pleural space
• Pulse oximetry or blood oxygen level test, to check how much oxygen is in your blood
• Bronchoscopy, a procedure used to look inside your lungs' airways

Treatment for pneumonia depends on the type of pneumonia, which germ is causing it, and how severe it is:

• Antibiotics treat bacterial pneumonia and some types of fungal pneumonia. They do not work for viral pneumonia.
• In some cases, your provider may prescribe antiviral medicines for viral pneumonia
• Antifungal medicines treat other types of fungal pneumonia

You may need to be treated in a hospital if your symptoms are severe or if you are at risk for complications. While there, you may get additional treatments. For example, if your blood oxygen level is low, you may receive oxygen therapy.

It may take time to recover from pneumonia. Some people feel better within a week. For other people, it can take a month or more.

Vaccines can help prevent pneumonia caused by pneumococcal bacteria or the flu virus. Having good hygiene, not smoking, and having a healthy lifestyle may also help prevent pneumonia.

NIH: National Heart, Lung, and Blood Institute