Pneumonia is a disease of the lungs and the respiratory system. The lung contains many small bulbs, or sacs, called alveoli. These help to take out oxygen from the air. In the case of pneumonia, these bulbs become inflamed. They fill up with a fluid which is called pus, and can no longer absorb as much oxygen as before. This makes it hard for the person with pneumonia to breathe. They may feel out of breath, or like they are drowning.  They may also feel pain when they breathe.  Sometimes people die of pneumonia, even when they go to a hospital and take medicine.

Background
Pneumonia can be caused by bacteria, viruses, fungi or parasites. It can also be caused by chemical or physical damage done to the lungs. Other illnesses, like alcohol abuse or lung cancer, can also result in pneumonia.

People with pneumonia usually have difficulty breathing. They may also cough, or have pains in the chest area. The treatment of pneumonia depends on how the illness was caused. If it was caused by bacteria, antibiotics can be used to treat it.

People of all ages can have pneumonia. The disease is dangerous. Many people die from pneumonia, especially old people, or people with a weak immune system. 

According to some research in 2010, in some parts of the world where people are very poor like India, Nigeria, and Pakistan, pneumonia was the cause of death for more children under the age of five than any other disease.  Statistically, however, for every 2,000 children in the developing world who die of pneumonia, only one child in the developed world dies from the disease.  This is because of differences in health care and because of differences in rates of breastfeeding, not because of differences in children.  Breastfeeding newborn babies greatly increases their ability to survive pneumonia, but some cultures think of breastfeeding as taboo.

Other types of pneumonia 

 Severe acute respiratory syndrome (SARS)
SARS is a highly contagious and deadly type of pneumonia which first occurred in 2002 after initial outbreaks in China. SARS is caused by the SARS coronavirus, a pathogen (disease-causing organism) that was not known until then. New cases of SARS have not been seen since June 2003.
 Bronchiolitis obliterans organizing pneumonia (BOOP)
BOOP is caused by inflammation (swelling or irritation) of the small airways of the lungs. It is also known as cryptogenic organizing pneumonitis (COP).
 Eosinophilic pneumonia
Eosinophilic pneumonia is invasion of the lung by eosinophils. Eosinophils are a particular kind of white blood cells. Eosinophilic pneumonia often occurs in response to infection with a parasite or after exposure to certain types of environmental factors.
 Chemical pneumonia
Chemical pneumonia (usually called chemical pneumonitis) is caused by chemical toxins such as pesticides, which may enter the body by inhalation (breathing in) or by skin contact. When the toxic (poisonous or harmful) substance is an oil, the pneumonia may be called lipoid pneumonia.
 Aspiration pneumonia
Aspiration pneumonia (or aspiration pneumonitis) is caused by accidentally inhaling oral (mouth) or gastric (stomach) contents into the lungs, either while eating, or after reflux or vomiting, and causing damage to the lungs.  The lung inflammation from these materials is not an infection but it can eventually cause one, since the material taken into the lungs may contain bacteria. Aspiration is a common cause of death among hospital and nursing home patients, since they sometimes cannot cough hard enough to protect their airways.

References

Other websites 
 Merck Manual: Pneumonia
 Merck Manual of Geriatrics: Pneumonia in the elderly
 British Lung Foundation 

Infectious diseases
Pulmonology