MODERN PSYCHIATRY suggests that Ebeneezer Scrooge was not a mean old git at all. He was just depressed. He exhibited classic signs of anhedonia - the inability to experience pleasure. He had very little appetite, experienced sleep disturbance and found other people's good cheer irritating.

But one doctor believes that actually he may not have even needed to see a doctor, let alone the Ghost of Christmas Past. A hearty dose of paracetamol might have done the trick. Dr Bruce Charlton, lecturer in psychology at Newcastle University, has produced a radical document, based on 18 months' research, that says that depression in many cases should be treated like a physical illness and not a mental one. Shortly to be published in the journal Medical Hypotheses, the paper precedes a new book challenging current psychiatric thinking, Psychiatry and the Human Condition, which will be published early next year.

Dr Charlton's theory questions the traditional belief that depression is a chemical disorder of the brain. He says the low mood that people experience is a secondary symptom. The primary symptoms he says, are the physical, flu-like ones.

"The symptoms of depression are similar to what is called `sickness behaviour', where the body slows down in response to illness so that energy is preserved to fight disease,'' says Dr Charlton. "When we get flu we lose our appetite, we stop moving so much, physical exertion becomes much harder, we go off sex. We feel tired all the time. It's an adaptive response to infection. It happens in animals too."

After flu it is normal to suffer anhedonia. "The mood lowering of depression is a consequence of having these symptoms over a long period of time," says Dr Charlton. And hardly a startling consequence at that.

"When I suffer depression," says Georgina, 33, "it does feel like a chicken- and-egg situation. It's hard to know which comes first, the sinking feeling or the inability to get oneself out of the sinking feeling. I lie in bed in the morning and I can't get up. Can't move, can't wash, can't eat. I know that if I did these things I might feel better but somehow I can't. And when you haven't moved all day the sensations are similar to flu.

There are several pieces of evidence supporting Dr Charlton's ideas.

1. As well as raising the immune system, cytokines, the hormones secreted naturally by the immune system when the body is under attack, can lower the mood. This is shown when people take Interferon (often for cancer), which raises cytokine levels; it is an anti-inflammatory but causes depression almost immediately.

2. The older anti-depressants or tricyclics are also known to be powerful painkillers - even in animals. So they can thus help flu as well as treat mood disorders.

3. Anti-depressants do not have an immediate effect on mood, but they may on the physical symptoms of depression. Mood improvement is a gradual process that relies on a build-up of emotions.

Christmas is a perfect time to test the new theory. If not even Home Alone 2 on the telly can cheer you up, try seeing what some painkillers can do - that is if you want to improve. My father is famous for his "Family Christmas Flu" that would begin with a very bad mood that would overcome him like a fog the week beforehand. Fluey symptoms would develop in earnest on Christmas Eve, forcing him to miss confession and midnight mass and instead curl up in bed with Jane Austen. Only on Boxing Day, when the relatives had gone home, would he begin to recover. Flu or depression? He will never say.