Pneumonia is an infection or inflammation of the lungs, where the air sacs (alveoli) of the lungs become filled with fluid. Pneumonia is very common and can be caused by a variety of factors. Most cases of pneumonia are either caused by bacteria or a virus, and are most likely to occur after a bout of influenza or a cold. However, there are many microorganisms that can cause pneumonia such as mycoplasms, fungi or chemical agents. Because of the variety of microorganisms that can cause pneumonia, in many cases of pneumonia the specific cause is never determined, even after extensive testing.

In the early 20th century, pneumonia was the leading cause of death in the United States. Pneumonia can now often be treated effectively with antibiotics. However, it is still a serious medical condition that may be fatal in some people. Pneumonia (along with influenza) is among the most common causes of death in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC).

Bacteria, viruses and other organisms frequently enter the body through the nose and mouth. In most healthy people, these microorganisms are prevented from reaching the lungs by a variety of defenses such as the immune system, the cough reflex and small hair-like structures called cilia that line the bronchial tubes. These germs are usually either completely destroyed or severely weakened by the immune system. Sometimes, if they have not been destroyed, the germs can remain dormant in the throat. If the immune system then becomes compromised, it is possible for the germs to become active and progress through the respiratory system into the lungs. After the germs are in the lung’s air sacs, they can cause the sacs to become inflamed and fill with pus and mucus while the immune system attempts to fight the infection. This makes it difficult to breathe comfortably and can stop oxygen from entering the bloodstream, which makes the rest of the body unable to work efficiently. 

Pneumonia may affect one lung or one section (lobe) of one lung. This is called lobar pneumonia. If the pneumonia affects sections of both lungs it is called bronchial pneumonia or double pneumonia.

Sometimes, pneumonia is classified according to criteria such as where or how it was transmitted or the severity of the symptoms. Some common classifications of pneumonia include:

• Community-acquired pneumonia (CAP). Pneumonia that is transmitted within the general community. Community-acquired pneumonia is one of the most common classifications of pneumonia.
  
  
• Hospital-acquired pneumonia. Pneumonia that is transmitted while in institutional (hospital) care (nosocomial infections). Hospital-acquired pneumonia is considered more severe than pneumonia contracted elsewhere. The strains of bacteria that are transmitted in hospitals are more likely to have developed antibiotic resistance and are difficult to treat. People who develop pneumonia in the hospital are also more likely to have a weakened immune system and so are less able to fight the infection.
  
  
• Aspiration pneumonia. Pneumonia that is contracted by breathing in toxic particles. If the particles break through the defenses of the respiratory system, they can cause infection in the lungs. Aspiration pneumonia is most often contracted after vomiting. People who vomit while unconscious are most at risk from aspiration pneumonia.
  
  
• Walking pneumonia. Pneumonia with mild symptoms. Mycoplasma pneumonia is often described as walking pneumonia because of its notoriously mild symptoms that rarely require substantial bed rest.