GO to any pop or rock concert and you are sure to be surrounded by people filming the show on their mobile phones. There was something sad and sobering, though, about the videos shot by fans at a recent David Cassidy concert. The footage showed him forgetting the words to some of his most popular hits – songs he has been singing for half-a-century. At one point, he dries up while his band plays on behind him, and he acknowledges what has happened with outstretched arms. He embarked on lengthy anecdotes that did not seem to make much sense. He also appears to fall off the stage. Shortly afterwards, Cassidy disclosed that he has dementia. He is only 66 years old.

He told People magazine that he was fighting the same disease that had afflicted his mother Evelyn, who died in 2012, aged 2012. His grandfather had also had dementia.

“I was in denial,” Cassidy said, “but a part of me always knew this was coming.” Of Evelyn he recalled: “In the end, the only way I knew she recognized me is with one single tear that would drop from her eye every time I walked into the room … I feared I would end up that way.”

He was also concerned that “people would mistake my alcoholism with my diagnosis”. He has now decided to stop touring as a musician in order to concentrate on his health and happiness. “I want to focus on what I am, who I am and how I’ve been without any distractions,” he said. “I want to love. I want to enjoy life.”

The news came as an obvious shock to his many fans but also to people who, like me, remember him from his heyday at the dawn of the 1970s. Then, he was a clean-cut, all-American, enviably good-looking guy in his early 20s. The girls at school loved him. Some went for the tartan-clad Bay City Rollers, but there was no doubt that Cassidy was the heartthrob du jour.

He had a good voice, too. He had always wanted to be a musician, but his acting abilities landed him roles in such US TV shows as Ironside, Bonanza and Marcus Welby MD before he became part of the Partridge Family, a musical sitcom in which he co-starred with his stepmother, Shirley Jones.

The Partridge Family, and his run of hit singles, made his name. He “became the biggest TV and performing phenomenon in history by the time he was 20”, says the Cassidy website. “He became the first personality to be merchandised globally. His likeness appeared on everything from posters to lunch boxes, comic books, toys, cereal boxes and almost anything else imaginable.” Over the next five years, it continues, membership of his official fan club exceeded that of Elvis Presley and the Beatles.

My knowledge of David Cassidy pretty much began and ended with those early 70s hits of his, though in later years, from time to time, there were glimpses of his latest success on stage: in Broadway, the West End, Las Vegas. His most recent appearance on TV was in an episode of CSI: Crime Scene Investigation, in 2013.

In 2014 he did a number of television interviews in which he spoke, with unusual frankness, of his drink problem, his brushes with the law, his stint in rehab. But when he was not on the road or the stage, he sought to raise awareness of dementia. As his website puts it, he became “an active spokesperson and advocate for various Alzheimer’s organisations since his mother suffered and ultimately died from severe dementia. He will be travelling the country speaking on the subject when not touring and performing in concert”.

His fans have responded to the news of his dementia with messages of love and support. “Praying for you, David” is a common refrain in the comments section of his Facebook page. One fan writes: “To David, the guy who was my first real crush. You still mean the world to me and so many others around the world. Your decision to share your diagnosis was a brave and scary thing to do. I wish there was a way to pay you back for all the music and great memories you’ve given me over the years.” To all of them, Cassidy has expressed his gratitude via Twitter.

Dementia, as Professor John Starr, director of Alzheimer Scotland’s dementia research centre at the University of Edinburgh says, has various causes. “Although the majority of early-onset dementia cases – people who have the disease under 65 when it starts – don’t have a specific hereditary cause, you are much more likely to have relatives who have a dementing illness when you are younger.

“These fall into two main categories. There’s a rare group of families with a specific genetic mutation of a gene that means, almost certainly, that you will get a form of dementia. The two commonest types are Alzheimer’s Disease [over half the people in Scotland who have dementia have this] and Behaviour Variant Frontotemporal Dementia, or FTD.

“We do know that there are genetic risk factors for dementia. If you have a first-degree relative who has dementia before the age of 75, it increases your own risk of getting dementia, usually Alzheimer’s Disease. If you have someone who gets it at 74, the risk increase is very small. If your mother got dementia at 54, there’s a much higher risk for you than if she got it at 64.

“This reflects [the fact that] the more genetic contribution to the condition, the more likely you are to get it at an earlier age. If you think of it as a mixture between having a genetic pre-disposition and then anything else – lifestyle, the environment – that would contribute to getting dementia – then the more genetic loading you have, the more likely you are to get it.”

It’s like throwing a loaded dice, and the more genetic loading it has, the more likely is it that your number will come up. “It doesn’t definitely mean that you’re going to get it,” Prof Starr says. “It’s just that you’re more likely to get it. From what we can infer from what David Cassidy is saying, I don’t think he has a definite gene that runs in the family. It’s just that the dice seem to be loaded against him.”

Dementia, however, is becoming more and more common. Between one in three and one in four of us will experience at it at some point in our lifetimes – because we are living longer, and because the risk increases considerably, the older we are.