What is pneumonia? Symptoms of bacterial infection that affected Kim Kardashian's son

Kim Kardashian revealed her son Saint, 2, is back at home and recovering on Tuesday after being treated in hospital for pneumonia.

The reality TV star praised the resiliency of Saint, and said that their “end of year was challenging”.

Pneumonia is a type of acute respiratory infection, that affects the lungs.

The lungs are made up of tiny sacs, known as alveoli, that fill with air when a healthy person breathes. But, pneumonia causes the sacs to become full of pus and fluid, according to the World Health Organization (WHO).

“Pneumonia is the single largest infectious cause of death in children worldwide,” said the WHO.

“Pneumonia killed 920,136 children under the age of five in 2015, accounting for 16 per cent of all deaths of children under five years old.”

The condition is usually caused by a bacterial infection, form the bacterial Streptococcus pneumoniae.

But, it can also be caught by a virus, a fungus, or even by simply breathing in a harmful substance, including smoke.

Symptoms of pneumonia can develop quickly, over a 24 to 48-hour period, according to the NHS.

Signs of the condition include difficulty breathing, a fever, chest pain, sweating, shivering and having a dry cough. The cough could be accompanied by a thick, yellow, green, brown or blood-stained mucus.

Coughing up blood, vomiting, wheezing, and headaches could also be a sign of pneumonia.

People showing typical signs of pneumonia should see a GP, while those experiencing severe symptoms – rapid breathing, chest pain or confusion – should seek urgent medical attention.

Ensuring good standards of hygiene could reduce the risk of spreading pneumonia.

Covering your mouth with a tissue when sneezing will help prevent germs spreading, the NHS urged.

Throw away tissues immediately after being used, and wash your hands regularly to stop germs transferring to other people.

In the UK, pneumonia affects about eight people in every 1,000 adults every year. The condition is most common in either the very young, or the very old, and generally occurs more often in the autumn and winter months.