Pneumonia

Source: MedlinePlus Health Topics, U.S. National Library of Medicine

Last synced 8/4/2026Status: primary verified

What is pneumonia? 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. What causes pneumonia? 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 Who is at risk for pneumonia? 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

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This information is for general education and is not medical advice. Always talk to a licensed U.S. clinician about your situation, medications, or treatment decisions.