Pneumonia is an infection of the air sacs in the lungs and is caused by bacteria, viruses or, rarely, fungi. Most cases of pneumonia are caused by bacteria, most commonly bacteria called streptococcus pneumonia but viral pneumonia is more common in children.
 
The lungs are made up of separate lobes – three in the right lung and two in the left lung. Pneumonia can affect only one lobe of the lungs or it may be widespread in the lungs. The condition can be classified by the area of the lung affected and by the cause of the infection. Anyone can develop pneumonia but some groups are at greater risk:

People who drink excessive alcohol
Patients in hospital
People who have had strokes.
Babies and toddlers - particularly those born prematurely
People who have had a recent viral infection - such as a cold or influenza 
Smokers
People with chronic lung conditions
People with suppressed immune systems

Pneumonia can develop when a person breathes in small droplets that contain pneumonia-causing organisms. It can also occur when bacteria or viruses that are normally present in the mouth, nose and throat, enter the lungs.