Pneumonia is a serious lung infection. Many cases of pneumonia are caused by bacteria or viruses. Fungi may also cause pneumonia, but this is less common.
You may also get pneumonia after another illness, such as a cold, flu, or bronchitis. Those most at risk include older adults, smokers, and people with long-term (chronic) health problems or weak immune systems.
Air travels in and out of the lungs through tubes called airways.
The tubes branch into smaller passages called bronchioles. These end in tiny sacs called alveoli.
Blood vessels surrounding the alveoli take oxygen into the bloodstream. At the same time, the alveoli remove carbon dioxide (a waste gas) from the blood. The carbon dioxide is then exhaled.
Pneumonia causes the bronchioles and the alveoli to fill with excess mucus and become inflamed.
Your body’s response may be to cough. This can help clear out the fluid.
The fluid (or mucus) you cough up may appear green or dark yellow.
The excess mucus may make you feel short of breath.
The inflammation and infection may give you a fever.
Symptoms of pneumonia can come without warning. At first, you may think you have a cold or flu. But symptoms may get worse quickly, turning into pneumonia. Symptoms can be different for bacterial and viral pneumonia. Common symptoms may include the following:
Severe cough with green or yellow mucus that doesn't improve or that gets worse
Fever and chills
Nausea, vomiting or diarrhea
Shortness of breath with normal daily activities
Increased heart rate
Chest pain or discomfort when breathing in or coughing
Excessive sweating and clammy skin
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