pneumonia

Imagine the lung as a sponge full of tiny air pockets that normally stay clean and open so oxygen can slip into the blood. In pneumonia, an infection floods some of those pockets with fluid, pus and inflammatory cells, so the sponge becomes soggy in patches. Air can no longer reach the soaked areas, which is why a person with pneumonia may feel breathless, cough up colored phlegm, run a fever and feel genuinely unwell.

Medically, pneumonia is an infection of the lung parenchyma — the alveoli and surrounding tissue where gas exchange happens — most often caused by bacteria such as pneumococcus, by respiratory viruses, or less commonly by fungi. The body sends white blood cells to fight the invaders, and the resulting inflammatory exudate fills the air spaces. On a chest X-ray this shows up as a white patch called consolidation; on examination a doctor may hear crackles and dull percussion over the affected zone.

Pneumonia ranges from a mild illness treated at home with oral antibiotics to a life-threatening one needing hospital oxygen and intravenous drugs, depending on the germ, the patient's age and any underlying disease. Doctors often classify it by where it was caught — in the community versus in hospital — because that strongly predicts which organisms are likely and which treatment will work. It remains one of the most common serious infections worldwide, but most healthy adults recover fully with appropriate care.

It is worth noting that a single white patch on imaging is not always infection: tumors, bleeding, inflammation and fluid can mimic it, so the picture is read alongside symptoms, examination and tests rather than on its own.