Other families had holidays in Spain - we had nervous breakdowns. Childhood was heaps of fun. An early memory is of my paternal grandmother coming through the front door sobbing. My father took long periods off work and grew a beard. He was "tired", my mother said. Why didn't he go to bed early, I wanted to know. Not that sort of tired, she answered. "Tired in his mind." It was only later, when my teenage sister got tired, too, that I learned about clinical depression.

Since then, there has never been a period in my life without somebody suffering from mental illness. Depression, anxiety, paranoia and psychosis: our family has seen it all. Specific relatives would be mortified to be mentioned, so I won't. Suffice to say, we have dodgy genes, as my sister and I call them. I have been fortunate with the particular cocktail I was handed. I hail from my mother's slightly eccentric but more robust line. I do my own weeping and wailing, but am essentially an optimist who can pour a stiff drink, pick herself up and get going again. I do understand that others can't, but that doesn't stop me getting furious about it.

Yes, I feel sympathy when those I love are ill, but I also feel rage. However much I read on the subject - if there were a GCSE in serotonin levels and neurotransmitters, I'd pass it tomorrow - however completely I grasp that it's not the patient's fault, there is still a part of me that is filled with resentment. It is hard when someone is suddenly lost to you. Sometimes, the urge to shake them is overwhelming.

I have a friend whom I see regularly. He's the avuncular advice-giver, the mentor, the sounding board. I bring my problems and angst and can safely rant while he makes a soothing noise. We have agreed that he likes to be needed and I want to be looked after myself for a change. It is a friendship that works. Until each spring comes, that is, and he, too, gets depressed. I'll know it has happened from the first syllable as he picks up the phone. He has all the classic symptoms - tiredness, lethargy, a feeling of dull hopelessness, a frustration because his brain simply will not work in the same way. He describes it as the light going out. Everything is suddenly grey and his mind is clouded.

It sounds awful, and of course it is. I am upset for him, but my sense of loss and fear is acute. I try to hide it. I go round, take flowers; I give him a hug, but my arms are stiff. I want him to get better quickly, so he doesn't suffer. I also want him better for me.

If one in four of us suffers from depression, then three-quarters of the population have a hell of a lot to put up with. It is a sad fact that we are more sympathetic to physical disease, but it is not hard to see why. Once, sitting in the bleak mental-health unit of the local hospital, I got talking to a woman whose partner was having a 12th session of ECT in the hope of being jolted out of a deep, debilitating despair. Comforting me in my guilt and helplessness over my own relative, she said, "Just give what you have to give, because you could throw your whole life in there. Depression is a black hole. It will swallow you up and still never be enough."

"When people are ill, they are selfish," my father reminded me recently. "They just can't help it."

"I know," I told him. And neither can I.