Spending a penny in space is expensive. The lavatory designed for Nasa's space shuttles cost $23.4m. Back on Earth, sanitation is a privilege not a right: 2.6 billion people don't have access to even the most basic latrine. As George points out: "four in 10 people live in situations where they are surrounded by human excrement" - a child dies from diarrhoea every 15 seconds. Sanitation, which we take for granted, adds 20 years to the average human life. George uncovers the often less-than-fragrant truth about human waste. Le Corbusier thought the lavatory "one of the most beautiful objects industry has ever invented". The Japanese agree: their "robo-toilets" (wash and blow-dry as standard) are the most advanced in the world. In India, George meets some of the 1 million Dalits who empty dry latrines by hand. In London and New York she explores the ageing sewers and is surprised at the "amount of beauty" underground. There are a lot of mobile phones, too - 850,000 are flushed down British loos every year. Scatological history has never been so fascinating.

The British may be renowned for loving lavatorial humour but, as George reveals in this eye-opening guide to the world's sanitary habits, human waste is no laughing matter. Each year, millions die in developing countries because of poor hygiene. George is an entertaining reporter who only occasionally digresses into excessive technical detail. She visits London's overwhelmed sewers, examines Japan's passion for hi-tech lavatories and reveals how sewage is used to fuel China's stoves. But it's her tales of the Indians and Africans who defecate in the open or in ramshackle latrines, consequently consuming several grams of one another's faeces a day, that really shock in this thought-provoking, character-filled book.

Mention Agincourt and English hearts stir with pride. The victory on 25 October 1415, by a ragged army of around 10,000 soldiers over a French army vastly superior in numbers, still evokes profound nationalistic feelings.

What is not often recorded, however, is the fact that half of England's archers fought while naked below the waist. Henry V's army had been ravaged by dysentery. Thus Voltaire concluded England had 'taken victory with its pants down'. Shakespeare, of course, makes no reference to this ailment among the medical complications that were 'had on Crispin's day'. It is not the most delicate of subjects, after all.

Nor have our sensitivities changed much over the centuries. Faeces, excreta-related diseases, diarrhoea and sanitation still tend to be avoided as dinner-table talking points. Terms for excrement remain our conversational taboo, as Rose George notes in this important book. 'Sex can be talked about. Death has once again become conversational. Yet defecation remains closed behind the words, all chosen for their clean association, that we now use to keep the most animal aspect of our bodies in the backyards of our discourse.'

And that is a shame, the author argues. As she notes, Gandhi observed that sanitation was more important than independence. And certainly, for those who lack it, the consequences are usually terrible, as George makes very clear. A total of 2.6 billion people today have no access to clean food or water and lead lives surrounded by human excrement, either in the bushes outside their villages or in their city streets. 'It is tramped back in on their feet, carried on fingers on to clothes, food and drinking water,' says George.

As a result, more than two million people, most of them children, die every year of diarrhoea triggered by faecally contaminated food or water, the equivalent of a child losing his or her life every 15 seconds. Faeces-related diseases kill more people than Aids, TB or malaria.

Yet we appear to lack the language even to discuss this toll or its causes. Hence George's book, a bid to get us talking about the excrement that is killing a third of the planet, a task, we should note, that does have some notable antecedents. Rudyard Kipling thought sewers were fascinating. 'I study 'em and write about 'em when I can,' he announced in 1886. Chekhov described the dreadful sanitation of the Russian isle of Sakhalin, while Freud wrote that humanity would be best advised 'to admit shit's existence and dignify it as much as nature will allow'.

So how does George fare against such august competition in her self-appointed task of bring the topic of sewage out of the water closet? Fairly well, I would say. Given the unappetising nature of her subject, her narrative is surprisingly tasteful and she successfully straddles a fine line between being scatological and silly and being humourless and self-righteous. In the process, we are taken on a grand tour of the sewers of London, whose 37,000-mile network dwarfs all rivals including New York's puny 6,000-mile system and Paris's piddling (sorry) 1,500 miles; we meet the workers who make Japan's deluxe, rim-heated, anus-showering super-lavatories; and we visit the World Toilet Organisation and its international college in Singapore. We discover that James 1 of Scotland was murdered by noblemen while hiding in his privy; that the average human bowel movement weighs 250 grams (about half a pound); and that we each spend, on average, three years of our lives going to the lavatory.

Most important, however, we learn of the profound benefits to be had from properly dealing with our own excrement. Most estimates suggest modern drains, sewers and lavatories have added 20 years to the average lifespan in the West and have brought more benefits than antibiotics, anaesthetics or the Pill. Those who lack these basic necessities face the prospect of disease and death. Bad hygiene and unsafe water cause one in 10 of the world's illnesses. Without sanitation, civilisation is impossible. Hence George's desire that 'we talk frankly about shit'.

 Robin McKie is The Observer's science editor

"The waste remains," William Empson wrote in one of his villanelles, "the waste remains and kills." The line might have served as an epigraph for Rose George's book, which looks at the toxic effects of poor sanitation and the heroic efforts of the visionaries who seek to improve it, from Joseph Bazalgette in 19th-century London to his heirs in the developing world today. By "waste" she doesn't mean the stuff that goes into wheelie bins or on to landfill sites; she means the unmentionables we flush down the loo - supposing we're lucky enough to have one.

"Shit" is her preferred usage, rather than excreta, faeces or stools. The topic invites euphemism or tittering toilet humour, but she has an answer to those who think it beneath their dignity: how a society disposes of its sewage tells you a lot about its economy, politics and religion, she says. Disgust may be an understandable reaction ("Oh! Celia, Celia, Celia shits," Jonathan Swift cried in horror), and the author is no coprophagist. But without rubbing our noses in it, she shows how the problems associated with shit are impacting on the Earth.

The statistics are horrifying: 2.6 billion people - 40% of the world's population - with no access to a toilet or latrine; 90% of our sewage discharged, untreated, into oceans, rivers and lakes; 10 million viruses and 1 million bacteria in each gram of faeces; every 15 seconds a child dies of diarrhoea caused by contaminated water. In the face of such facts, polite avoidance of the subject seems irresponsible. But persuading people to discuss it openly (as they have learned to with formerly taboo subjects such as sex and death) is a struggle. An advertisement for Velvet toilet tissue in 2002, with artily photographed bare bottoms and the slogan "Love your bum", was the second most complained-about ad that year. Celebs will do their bit for WaterAid but not for sanitation. Even those in the industry are shy about admitting it, for fear the rest of us will back away.

With no qualifications except an intrepid spirit and a resolve to look and learn, George visits some of the dirtiest places in the world. She goes down among the turds in London's sewers, finding the air surprisingly wholesome but the number of employees or "flushers" (a mere 30-odd for the whole of the capital) alarmingly small. She inspects school lavatories in South Africa (where they exist, and are clean, the attendance rate is higher), mingles with untouchables in India (though officially they no longer exist), samples a biogas stove in faeca-philiac China (it runs off converted human excrement), watches latrine pits being emptied in Dar es Salaam (sledgehammers and kerosene do the trick), and handles the sludge used as a fertiliser by American farmers ("It looks and feels like a crumbled brownie"). Less gruellingly, she samples state-of-the-art lavatories in Japan, which can check your blood pressure, play music and automatically put the seat down for you, but which have had no more success as an export to the US than the bidet did - to middle America, anal washing seems pervy and louche. Australia is about the only place George doesn't visit, though she does talk to Aussies in the business. Whenever there's a toilet conference, be it in Moscow or Bangkok, there she is with her notebook and an ear cocked for absurdity.

Along the way, she explodes a number of myths. That we in the west are well sorted in terms of "watsan" (water and sanitation), for instance. Not so: recent years have seen outbreaks of disease from Galway to Milwaukee and catastrophic sewer overflows in London and New York. Nor are developing countries as indifferent to hygiene as popularly alleged: George meets countless evangelists waging war on ignorance, not least in India, where local initiatives have brought sanitation to villagers who used to squat in the bushes or next to ponds. The dream is to create a continent that's ODF, open defecation-free - no more bottoms discharging on to the railway in the slums of Mumbai and elsewhere.

Though charmed by the evangelists, George is sceptical of their claims to lasting success. And having listened to the arguments on both sides, she can't decide where she stands on biosolids: are the Americans and Chinese who use human shit as fertiliser noble recycling pioneers or dangerous polluters? The book seems repetitive at times, because of its structure - part travel book, part reportage, part polemic - and it's a pity the notion of waste wasn't broadened out. It's an invaluable contribution nevertheless.

"Shit's shit," Andrew Motion once wrote (poets seem better at facing the issue head-on), "and what we desire in the world is less, not more, of it." Less won't happen, with the global population expanding at its current rate. But given that "sanitation is one of the best investments a country can make" (good for health, tourism and the economy), it's bewildering how little is spent on it - whether in Pakistan (which spends 47 times more on its military, despite 120,000 diarrhoea-related deaths a year) or in the UK (where sewage systems in many cities haven't changed since Bazalgette). Rose George doesn't pretend to have the answers. But as she prods, pesters and provokes, she proves an excellent shit-stirrer.

 Blake Morrison's South of the River is published by Vintage