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Bacterial Pneumonia

Pneumonia is an infection of the lungs. People with pneumonia usually complain of coughing, fever, shortness of breath, and chest pain.

Your body's immune system usually keeps bacteria from infecting your lungs. In bacterial pneumonia, bacteria reproduce in your lungs, while your body tries to fight off the infection. This response to bacterial invaders is called inflammation.
When the inflammation occurs in the alveoli (microscopic air sacs in the lungs), they fill with fluid. Your lungs become less elastic and cannot take oxygen into the blood or remove carbon dioxide from the blood as efficiently as usual.
When the alveoli don't work efficiently, your lungs have to work harder to satisfy your body's need for oxygen. This causes the feeling of being short of breath, which is one of the most common symptoms of pneumonia. Inflammation causes many of the other symptoms, including fever and chest pain.
Pneumonia can be very serious, because it directly interferes with your body's ability to exchange carbon dioxide and oxygen.
Pneumonia is different in this way from acute bronchitis, which is another disease that can cause fever, cough, chest pain, and shortness of breath. Bronchitis is caused by inflammation in the air passages (called bronchi) leading to the alveoli, not the alveoli themselves. Sometimes it is very difficult, even for a doctor, to tell pneumonia and bronchitis apart. The symptoms and physical examination can be identical. Sometimes a chest x-ray is the only way to tell pneumonia and bronchitis apart.