An elderly friend of mine was ashamed of having cancer.

It sounds bizarre, doesn't it? I mean why shirk from acknowledging a life-threatening illness? But there was a time when such misfortune was hidden away because of the stigma. "That was how things were then," she told me.

We saw secrecy when Aids was first identified. Fear of the unknown made pariahs of early sufferers. Remember how Princess Diana broke the taboo by being photographed shaking the hand of a man with Aids. Now that we understand the virus, now that retroviral drugs are controlling it, the stigma has receded.

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There's another illness as commonplace as cancer that we still seem to cringe from. It's all around us but the symptoms are invisible and the sufferers are too often silent. It is depression.

During the past few days it seemed to be in every newspaper. Alasdair Gray, the writer whose novel Lanark is a landmark of Scottish literature, told how he had his head in a gas oven when a visit from his father aborted the suicide attempt. He tried three times.

Steven Purcell charted the breakdown which precipitated his fall from being the youngest man to run Glasgow City Council. He took himself into counselling to disperse his demons.

And Celtic manager Neil Lennon has again spoken about his battles with depression while he was a player. He is appearing in an educational film Mind Games: Mental Health in Scottish Football because he believes open discussion of the illness is vital.

Former Livingston player Iain Russell, a fellow sufferer said: "There were days when I hoped I would never wake up."

Then, of course, there were reports about fashion designer L'Wren Scott whose suicide has devastated her partner Mick Jagger.

Do high-profile people have a propensity to suffer? Is there too great a discrepancy between the image manufactured for public consumption and the real person?

Nigella Lawson once said: "There is a vast difference between how things seem from the outside and how they feel on the inside."

You don't have to be famous for that to be true, though it's the famous we hear about. The world of sport is peppered with young men who struggled in silence for too long. Cricket star Andrew Flintoff said his depression was "an affliction made all the worse because in sport you dare not speak its name".

We saw him as ebullient and confident yet at times he was so low he didn't want to get out of bed.

Football hardman Vinnie Jones said depression would have been seen as weakness. He reached a point of taking a gun into the woods to end his life. Boxer Ricky Hatton suffered for three years after losing an important fight. He turned to drink and would end the night sitting sobbing in the corner of a pub.

Depression is both common and hidden but it is no respecter of persons. We are almost as likely to suffer from it as we are to catch flu. The difference is we have a healthy attitude to catching flu.

It's nothing new. Depression was the lot of Dostoyevsky, Mozart too. It dogged Winston Churchill and blighted Marilyn Monroe. Today one in five of us gets depressed at some stage in our lives. So why would the rich and famous escape? Why would any of us? And what is it that we are ashamed of?

According to Action on Depression, up to 500,000 people in Scotland could be suffering depression right now - and half will be struggling without treatment.

The World Health Organisation predicts depression will become the second biggest illness worldwide. How can we be so reticent about acknowledging it when it is so clearly part of the human condition?

Perhaps it is because depression brings with it hopelessness, worthlessness and despair. It blots out the sun and convinces us it will never shine again - at least not on us - so what's the point of seeking help? That's the mindset but the reality is we can get well again.

I feel fortunate to have experienced depression only once. I'm not talking about the blues; I'm talking about a spring day many years ago now when I took my small children to see new-born lambs. As I sat apart watching their excited delight I found myself thinking "What do they have to do with me anyway?"

I shocked myself into awareness. The thought was so alien; so far removed from my real response to family life that it made me take stock.

I'd been physically unwell, I had a new, stressful job and I'd been in a dreadful car crash. I was buckling and I hadn't recognized the fact.

As I sat there I acknowledged sleepless nights and a sense of dragging through days when every small domestic chore felt insurmountable and there seemed to be a glass wall between me and other people. Nothing pleased me. No prospect excited me. But it took the visit to the lambs, the emotional barrier between me and my children, before I named it as depression.

The next day I went to my GP. With a counsellor I talked myself back to normality. Like I say, I was fortunate. The black dog never did return.

Others are not so lucky. Depression is common during teenage years and when people are grieving. For some people it is seasonal. Women can get depressed after the birth of a baby. Sometimes it just descends without apology or excuse. Always it has to be battled, like any other illness. Secrecy or a sense of shame doesn't help.

Al Gore's ex-wife Tipper said of her experience: "You can't will your way out of that or pray your way out or pull yourself up by your bootstraps. You really have to go get help, and I did."

It is a real step forward to have men like Lennon acknowledge they have endured this illness. It explodes the myth that emotional distress is unmanly - that strong men don't feel hurt. In our celebrity-driven society it is salutary to know fame and fortune is not a bulwark against real human experience. Just as the famous suffer the same physical illnesses as the rest of us, so they suffer the same mental ones.

And just as we wouldn't feel the need to pretend we didn't have a broken leg, my hope is we soon reach the stage where we don't try to hide a chronically low mood.

Richard Monaghan, a volunteer with See Me, Scotland's national programme to end mental health stigma and discrimination, dipped into depression when he set up in business for the first time. He was prescribed medication and visited a psychiatrist weekly but found it hard to go into public places in case people recognised his condition.

Stigma, he thinks, makes sufferers less likely to speak out. This, in turn, isolates them.

Let's remind ourselves that there could be half a million Scots suffering from this debilitating illness right now. If it damaged bodies, we'd be pouring resources into supporting them while finding a cure. That we don't do as much for wounded minds is scandalous.