Absolutely nothing?

That's the impression. The very idea of it makes some of us react with revulsion. It sounds like an obscenity, an insult, an affront to everything clean and pure - something that must be energetically fought, using every weapon available.

But what is it?

The OED tells us that it's 'rotting or purulent matter ... dirt esp. of a disgusting kind', among other things.

We're not talking good, honest, 'clean' dirt - or potting compost - here. It's dirtier than that! It's the kind of thing that could give you a disease, if you touched it - perhaps even if you approached it too closely. The idea that a child would pick up a handful and shove the festering umska straight into its mouth (which, of course, any curious infant would if they could get within grabbing distance), might be enough to make a conscientious mother feel quite faint.

And the world is full of it. Step outside the sterile bubble of your home or office and there it is - almost everywhere you look - a threat to human health and well-being. It's a wonder we've survived so long... isn't it?

We are bombarded with advertisements for products that claim to be able to protect us, our children and our homes from the dangers inherent in dirty hands, teeth, hair, clothes, floors, sinks, work surfaces, toilets - anything we might touch or might touch us, and thereby contaminate us with nasty, dangerous filth. News programmes and documentaries reinforce our fears of contamination: 'scare stories' about hospital acquired infections, legionnaires' disease misting down onto our streets from air conditioning units, children getting toxoplasmosis from cat faeces in their own gardens.

Perhaps there's a natural human inclination to exaggerate danger on the one hand, and an equally natural inclination to over-react to perceived danger on the other. Some diseases will kill you. Better safe than sorry! There was a time, not so long ago, when people feared catching AIDS from lavatory seats, cups and cutlery. Anyone unfortunate enough to contract leprosy, was no longer called a 'person' - they were called a 'leper', regarded as 'unclean' and segregated from the rest of society. Though leprosy itself is not very infectious, the fear of it is.

There seems to be a definite tendency to extremism in the human psyche. So maybe it's not surprising that, in our efforts to be clean and to be seen to be clean, we may be going too far. There must be some moderately grubby state on the continuum between filth and squalor at one extreme and sterility at the other, where we could be mentally and physically comfortable.

What's wrong with being clean?

We don't want to contract diseases or host parasites¹. A clean environment minimises our risk of coming into contact with things that might infect us. But there are other risks to human health, that may be caused or exacerbated by our obsession with the removal of dirt. The 'hygiene hypothesis' suggests that allergies which were hardly even heard of, only a generation ago, are increasing as a result of our very successful efforts to isolate ourselves from the things that evolution has equipped our immune systems to fight.

The number of children suffering from peanut allergy has, reportedly, doubled in the last decade. Cases of asthma have trebled in the last 20 years. A too-clean environment is also blamed for increases in eczema, hay fever - and even diabetes.

In a culture that is so obsessed with cleaning and washing, it's unsettling to read that we may also be soaping and scrubbing away protective layers of skin together with the filth, and thereby enabling particles to penetrate and cause allergic reactions.

Don't we need a more balanced and considered approach?

So, what's going on with the immune system?

The immune system has been evolving for billions of years - ever since a small living thing first had the temerity to attempt to take up residence inside a larger living thing. The parasitic way of life has been a great success, but only because parasites are incredibly adaptable. We still host these parasites, just as our ancestors did. And we have been very successful too - because we are also extremely adaptable.

Our immune systems are sophisticated intruder alarms and energetic war machines - always on the look-out for old enemies and new. And, of course, old enemies in disguise. The enemy is sly, deceitful, evasive - from the 'simplest' virus to the most complex eukaryote. They change and evolve, duck and dive, hide where the forces of the immune system can't find them. So the immune system has to be constantly vigilant, sensitive to changes, ready to recognise anything even slightly unusual or out of place and it has to keep records.

We have been shaped by our environment, including the parasites and diseases that plagued us in the past. It might not be overstating the case, to say that our parasites made us what we are today. We certainly would not be able to survive without the busy, efficient immune system that has continually modified itself to counter every trick and ploy of our old adversaries. Every one of us alive today, comes from an almost unimaginably long line of ancestors - stretching far back to the earliest creatures - whose immune responses enabled them to survive long enough to reproduce.

Now this engine of military might finds itself in the unnaturally 'clean' environment of the modern western world. It's almost as though there's been an attempt to sterilise it out of a job, and a belief that it's right to do so. We can't expect the immune system to quietly retire, atrophy and die, because we don't need it any more. Even though some of us have made our environment so sterile that our bodies hardly ever encounter a parasite, they are still there and we still need our defences.

The problem may be that, stimulated or not, the immune system never sleeps. It needs a job. While there are no real enemies, it may have to keep itself in training by engaging in battle with less harmful materials like pollen or dust mites - or even the useful materials that any normal animal would regard as food, like peanuts or shellfish.

It appears that our fastidious cleanliness may have triggered the law of unintended consequences.

Can it be fixed?

What we need, is a modicum of challenge - something to keep our immune systems focused on the real enemy. We need some muck in our lives. Not too much. Just enough to keep us fit: a little something to inoculate us, so to speak - to prevent gratuitous attacks by the body's defences against itself, or innocuous substances that represent no threat.

Some of the answers suggested by science might be enough to put you off your breakfast. A few decades ago, it was noticed that people infected with hookworms didn't suffer from conditions ranging from asthma to Crohn's disease. So some courageous British scientists at Nottingham University, infected themselves with the larvae of these worms to see whether they would suffer any harm. They survived and the worm 'cure' caught on.

Another idea, not quite so repellent as the hookworms, is to use bacteria found in soil to make a vaccine. And, far more agreeable than either of those, is the suggestion that children who grow up with dogs and cats have a reduced risk of developing allergies.

The simplest suggestion of all might be to seize the excuse to be just a little bit more of a slob. Ignore the media scares and the adverts for cleaning products. Spend your money on something more indulgent. Don't worry. Be happy!³