Pott's disease is a presentation of extrapulmonary tuberculosis that affects the spine, a kind of tuberculous arthritis of the intervertebral joints. It is named after Percivall Pott (1714–1788), a London surgeon who trained at St Bartholomew's Hospital, London. The lower thoracic and upper lumbar vertebrae are the areas of the spine most often affected. Scientifically, it is called tuberculous spondylitis and it is most commonly localized in the thoracic portion of the spine. Pott’s disease results from haematogenous spread of tuberculosis from other sites, often pulmonary. The infection then spreads from two adjacent vertebrae into the adjoining intervertebral disc space. If only one vertebra is affected, the disc is normal, but if two are involved, the disc, which is avascular, cannot receive nutrients and collapses. The disc tissue dies and is broken down by caseation, leading to vertebral narrowing and eventually to vertebral collapse and spinal damage. A dry soft tissue mass often forms and superinfection is rare.