Pneumonia is an infection of one or both of the lungs caused by bacteria, viruses or fungi.
It is a serious infection in which the air sacs fill with pus and other liquid.
Pneumonia is classified as severe when the heart, the kidneys or the circulatory system are at risk of failing, or if the lungs can no longer take in enough oxygen.
According to Johns Hopkins Medicine, there are more than 30 different causes of pneumonia grouped by the cause.
However, the main types of pneumonia are bacterial, viral and mycoplasma pneumonia.