In September (16) it was announced that the 74-year-old funnyman was suffering from primary progressive aphasia - a type of dementia affecting the brain's frontal lobe.
Eric Idle has revealed that Jones' fellow Pythons have known he was suffering from the disease for a long time, and helped him through the group's performance's at London's O2 Arena.
"It's been coming on for about five or six years," Idle told Britain's Radio Times magazine. "We've all known about it. I was only happy that we managed to do the Monty Python shows at the O2 while we could still get him through it. We said, 'Look, Terry, don't worry, we're going to get you through this, we're all in this together.'"
Jones's diagnosis was made public by his representative ahead of the he British Academy of Film and Television Arts (BAFTA) Cymru (Wales) Awards, where he received a lifetime achievement award.
His spokesperson said the illness "affects his ability to communicate and he is no longer able to give interviews. Terry is proud and honoured to be recognised in this way and is looking forward to the celebrations."
Idle said that his old comedy colleague is still himself despite his battle with the incurable disease, adding, "It doesn't seem to me that he's unhappy. He hasn't forgotten who he is, yet. Terry's still here. He's not gone."
Python original Michael Palin, 73, takes Jones on trips out to the pub, while Idle has also seen his friend in recent months.
All five surviving members of the group reunited at the Tribeca Film Festival in New York last year (15).
Idle said that seeing his friend's dementia symptoms become more obvious when he was with him in the U.S. has made him glad Jones' family have gone public with his diagnosis.
"He was on stage there, but he was having trouble stringing sentences together and it was becoming noticeable - so I'm glad they came out with it and acknowledged his illness in public," Idle said.
Jones was responsible for directing or co-directing all three Monty Python feature films, with his last major film the 2015 comedy Absolutely Anything, starring Simon Pegg and Kate Beckinsale.