Do you think diseases can be divided up into those it's OK to laugh at, and those it isn't? So cancer isn't funny, but bunions are?

Should I be laughing at Keeping Mum?

By which you mean the Thursday evening sitcom on BBC1?

Yes. I roared along to it until Gerard Gilbert suggested that we were laughing at someone with Alzheimer's disease ...

Mmm. Well, I've only watched it the once ...

And did you laugh?

No. I was too busy trying to diagnose Stephanie Cole.

So she may not have Alzheimer's disease?

No.

Great. Does that mean I can start laughing at her again?

It's up to you. Ms Cole's character has, at the very least, some diminution in her concentration and memory. This happens to all of us as we age, and although we may joke that we are going demented, it's far more likely that we are suffering from "benign senescent forgetfulness".

Come again?

See, it's happening to you. BSF may be normal, or result from minimal pathological change in the brain. It causes annoyance and anxiety but is a far cry from brain failure, in which the brain as a whole functions abnormally because of widespread tissue damage.

Due to Alzheimer's?

That's one cause, but brain failure can also be caused by multiple infarcts, Huntington's chorea, Creutzfeldt-Jakob disease, normal-pressure hydrocephalus, syphilis, Parkinson's, Aids, malnutrition, hypothyroidism and a whole host of others.

So if Stephanie Cole has undiagnosed brain failure, I could be laughing at any of these diseases?

Yes. Does that make a difference?

Sort of. Laughing at an underactive thyroid seems more excusable than laughing at Alzheimer's.

So you think diseases can be divided up into those it's OK to laugh at and those it isn't?

Well, I mean cancer and cerebral palsy are obviously never funny, but bunions are.

I see. The legitimacy of the target depends on the severity of the illness?

Yes.

So it may be all right to laugh at someone in the early stages of dementia, when the symptoms are relatively mild, but not in the latter stages, when she's violent, confused and incontinent of urine and faeces.

Well, no. But surely not everyone gets it that bad?

But they do. This horrific disease is invariably progressive, with a mean survival of five to seven years.

So it's not a sitcom that's going to run and run?

Let's hope not.

Well, I shan't laugh at it again.

Suppose Stephanie Cole the actress really does have Alzheimer's disease, and wants to laugh about it through her sitcom character. Is that OK?

I'm not sure.

There's a comedian with cerebral palsy who laughs at his condition, to much critical acclaim, so why not Alzheimer's?

Mmm. I suppose it depends on whether she knows what she's doing.

Precisely. Satirising your own pathology is fine until you reach the stage of complete mental incompetence, when it would be unkind to give you air-time.

But Stephanie Cole doesn't have Alzheimer's.

Not yet, no. But she is one of our most loved and revered actresses, which is why she's managed to gloss over the controversy in a passable Bristol accent.

I can't think why I ever found it funny.

Yes, you can. For some perverse Darwinian reason, humans have a basic need to laugh at the disadvantaged, and you can't get more disadvantaged than brain failure. Indeed, the common manifestations of this group of diseases are manna from heaven for a desperate sitcom writer.

Such as?

Such as leaving the gas turned on, boiling kettles dry, dressing with pants on the outside, making excessive phone calls, talking about dead people as if they were still alive, wasting money, wandering the streets in your nightwear, misidentifying your family, talking and gesticulating to people who aren't there, conversing with the TV, sexual exposure, refusing to go to the toilet, soiling the clothes and hands with excreta ...

That's enough, thank you.

You're right. Analysing sitcoms is pointless. Far better to sit back and let it bypass your brainn

Correction: Last week, I was erroneously credited with dispatching simulated patients to 23 Norwegian GPs. This fine piece of research was in fact the work of Jan-Joost Rethans and Lars Saebu, and was published in the 'British Medical Journal' on 19 April.