What causes Pneumonia ? 

Many germs can cause pneumonia. Examples include different kinds of bacteria, viruses, and, less often, fungi.
                
Most of the time, the body filters germs out of the air that we breathe to protect the lungs from infection. Your immune system, the shape of your nose and throat, your ability to cough, and fine, hair-like structures called cilia (SIL-e-ah) help stop the germs from reaching your lungs. (For more information, go to the Diseases and Conditions Index How the Lungs Work article.)
                
Sometimes, though, germs manage to enter the lungs and cause infections. This is more likely to occur if:
                
Your immune system is weak
                
A germ is very strong
                
Your body fails to filter germs out of the air that you breathe
                
For example, if you can't cough because you've had a stroke or are sedated, germs may remain in your airways. ("Sedated" means you're given medicine to make you sleepy.)
                
When germs reach your lungs, your immune system goes into action. It sends many kinds of cells to attack the germs. These cells cause the alveoli (air sacs) to become red and inflamed and to fill up with fluid and pus. This causes the symptoms of pneumonia.
                
Germs That Can Cause Pneumonia
                
Bacteria
                
Bacteria are the most common cause of pneumonia in adults. Some people, especially the elderly and those who are disabled, may get bacterial pneumonia after having the flu or even a common cold.
                
Many types of bacteria can cause pneumonia. Bacterial pneumonia can occur on its own or develop after you've had a cold or the flu. This type of pneumonia often affects one lobe, or area, of a lung. When this happens, the condition is called lobar pneumonia.
                
The most common cause of pneumonia in the United States is the bacterium Streptococcus (strep-to-KOK-us) pneumoniae, or pneumococcus (nu-mo-KOK-us).
                
Lobar Pneumonia
                

                
Another type of bacterial pneumonia is called atypical pneumonia. Atypical pneumonia includes:
                
Legionella pneumophila. This type of pneumonia sometimes is called Legionnaire's disease, and it has caused serious outbreaks. Outbreaks have been linked to exposure to cooling towers, whirlpool spas, and decorative fountains.
                
Mycoplasma pneumonia. This is a common type of pneumonia that usually affects people younger than 40 years old. People who live or work in crowded places like schools, homeless shelters, and prisons are at higher risk for this type of pneumonia. It's usually mild and responds well to treatment with antibiotics. However, mycoplasma pneumonia can be very serious. It may be associated with a skin rash and hemolysis (the breakdown of red blood cells).
                
Chlamydophila pneumoniae. This type of pneumonia can occur all year and often is mild. The infection is most common in people 65 to 79 years old.
                
Viruses
                
Respiratory viruses cause up to one-third of the pneumonia cases in the United States each year. These viruses are the most common cause of pneumonia in children younger than 5 years old.
                
Most cases of viral pneumonia are mild. They get better in about 1 to 3 weeks without treatment. Some cases are more serious and may require treatment in a hospital.
                
If you have viral pneumonia, you run the risk of getting bacterial pneumonia as well.
                
The flu virus is the most common cause of viral pneumonia in adults. Other viruses that cause pneumonia include respiratory syncytial virus, rhinovirus, herpes simplex virus, severe acute respiratory syndrome (SARS), and more.
                
Fungi
                
Three types of fungi in the soil in some parts of the United States can cause pneumonia. These fungi are:
                
Coccidioidomycosis (kok-sid-e-OY-do-mi-KO-sis). This fungus is found in Southern California and the desert Southwest.
                
Histoplasmosis (HIS-to-plaz-MO-sis). This fungus is found in the Ohio and Mississippi River Valleys.
                
Cryptococcus (krip-to-KOK-us). This fungus is found throughout the United States in bird droppings and soil contaminated with bird droppings.
                
Most people exposed to these fungi don't get sick, but some do and require treatment.
                
Serious fungal infections are most common in people who have weak immune systems due to the long-term use of medicines to suppress their immune systems or having HIV/AIDS.
                
Pneumocystis jiroveci (nu-mo-SIS-tis ye-RO-VECH-e), formerly Pneumocystis carinii, sometimes is considered a fungal pneumonia. However, it's not treated with the usual antifungal medicines. This type of infection is most common in people who:
                
Have HIV/AIDS or cancer
                
Have had an organ transplant and/or blood and marrow stem cell transplant
                
Take medicines that affect their immune systems
                
Other kinds of fungal infections also can lead to pneumonia.