Pneumonia is infection of the lungs.
The infection is in the lung tissue and the tiny air sacs in the lungs where the blood receives oxygen from air that is breathed in. It is extremely common in the UK and can range from a mild illness to a fatal one, especially if the person is already sick or in hospital. Pneumonia is not a single disease but refers to many different infections, each caused by a different micro-organism. Pneumonia can be caused by viruses, bacteria, fungi or bacteria-like organisms called mycoplasma . Pneumonia can also be caused by the inhalation of substances into the lungs such as caustic chemicals, food or vomit.