Pneumonia is a serious infection of the lungs that causes the air sacs in the lungs to fill with pus and other liquid. The infection may be caused by any of the following causative agents: bacteria, virus, fungi, and mycoplasma.
In general, pneumonia is divided into two types: community acquired or hospital or nursing home acquired.
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You may get pneumonia simply because you encounter an organism that you breathe into your lungs. Whether or not you get sick depends on the contagiousness of the organism, the number of organisms you inhaled, and the ability of your immune system to fight infections.
In the United States, pneumonia is a very common illness, especially among people who are elderly.