Seeing the world is one of my great pleasures, and I hate to think what proportion of my income is spent on holidays.

It all began when my wife realised how poor I was at relaxing unless frog-marched far from my computer, books and never-ending in-tray. But there's another reason for wanting to see the world now, which was actually summed up beautifully by an old friend last year. We had returned from to Borneo, when Carol phoned: 'I wanted to check you're okay,' she said. 'You're travelling so much we thought one of you must have a terminal illness and you're trying to squeeze everything in while you can.'

She was partly right. Although neither of us has a terminal illness apart from life itself, we are squeezing as much into our lives as we can, for how often do you see patients whose future plans are suddenly halted by unexpected disease? As someone whose GP father died when he was 61, and whose GP brother died at 38, I have probably been more aware than most of the potential for disaster - which is my excuse for taking a holiday at every possible opportunity.

But not everyone thinks like this. Brenda and Matthew have been my patients for about 30 years, and I doubt if I have ever known a more hardworking pair of people. Brenda had a hair-dressing business, travelling to clients' homes and building up a devoted clientele of the elderly, disabled, and agoraphobic. Meanwhile, Matthew busily made a pile in the City.

They have both had their fair share of middle ranking illnesses over the years - none of the premiership conditions like diabetes or cancer, but enough for me to know them reasonably well.

They were struck by the fact that my consulting room walls display a constantly changing selection of photographs of different parts of the world. Matthew explained: 'When I'm 55 we are going to retire and see the world. We'll come back and gloat then.'

He was 55 a couple of months ago. There was something tragically inevitable about the significant haemoptysis he had just after his birthday. And a year or so ago, Brenda started to consult me with a foot drop that was difficult to diagnose. Neurology has never been one of my strong points (a bit like orthopaedics, cardiovascular medicine, and respiratory problems have never been my strong points) but I knew enough to work through the options.

There didn't seem to be any reasons for damage to her popliteal nerve, no spinal symptoms, no sign of peripheral neuropathy, no diabetes or alcohol excess, or vasculitis, or Guillain Barre - no sign of any explanation at all.

And so I sent her to a neurologist. She had every possible investigation, and all were negative. The neurologist was stumped.

And then, a few days after Matthew had been in to tell me about his haemoptysis, Brenda came to ask if I thought this curious twitching of her muscles was anything to worry about. And yes - as you have doubtless realised - she had typical fasciculation, and the penny finally dropped: motor neurone disease.

I guess this hasn't been a very cheery column. Brenda and Matthew won't get their world tour. Sometimes life seems desperately unfair. But I have to say their story has reinforced my own determination to do it all.

Our trip to Peru in the autumn has now been booked.