Primary progressive aphasia (PPA) is a rare type of dementia which affects speech and communication.
In 2016, it was announced that Monty Python’s Terry Jones had been diagnosed with the condition. He died on Wednesday aged 77.
It is a form of frontotemporal dementia (FTD) where symptoms get progressively worse over time as the brain tissue which is important for speech and language deteriorates.
It is caused by clumps of abnormal protein forming inside brain cells, mainly in the front and side of the brain, that control language and behaviour.
These are thought to stop cells working properly by damaging them.
While dementia usually strikes over the age of 65, FTD tends to start at a younger age. Most cases are diagnosed in people aged 45-65.
The first symptoms of PPA are problems with speech and language, such as struggling to find the right word or remember somebody’s name.
Speech can become slow and hesitant, with sufferers reluctant to join in conversations.
As the condition progresses, other symptoms can include changes in personality, memory loss and movement difficulty.
During work to publicise the condition, Jones revealed that he could no longer write.
His Python colleague Micahel Palin told The Observer: “He knew exactly what was affecting him and he wanted to share that knowledge – because that is the way that Terry is.
“FTD may cause loss of inhibition, but Terry was never very inhibited in the first place.”