The volunteer farmers who bare their skin throughout the night and offer their blood to the mosquitoes of Pailin are taking grave risks for a good reason. Armed with a flashlight, some cotton wool and a test tube each, and with their trouser legs rolled up, it is their low-tech duty to wait until an insect, attracted mostly by the carbon dioxide in perspiration or breath, alights on their skin, and then to capture it for the laboratories of Phnom Penh, where its disease-bearing parasite levels can be assessed.

In this remotest district of Cambodia, tight up against the Thai border on the northern slopes of the Cardamom mountains where the Khmer Rouge fought their final battles, and where some of their cadres are now senior figures in the administration, the mosquitoes are among the world's most deadly.

A single bite and any of these eight human baits, working in four-hour shifts, could become the bone-aching, haemorrhaging victims of dengue fever, which is at its most virulent during this season of monsoons and which, though not normally lethal except for children and the very old, is incurable. According to Richard Veerman, my Médecins Sans Frontières (MSF) host and its head of mission in Cambodia, who has himself only just recovered from the fever, "You can do nothing about it but sit out the pain and hope not to die."

But it is not dengue that should worry me most, when I sit down on the boggy ground in the tiny village of Pang Rolim to join the volunteers and bare my own leg to the mosquitoes. Nor is it the deadly russell's vipers or the cobras that are common in the soya-bean fields behind us. Nor is it even the undetected and undetonated landmines that, according to a recent Unicef report, make these farmlands "one of the most dangerous places in the world".

What should have kept our legs covered is the knowledge that the anopheles mosquitoes of Pailin or, more specifically, the single-cell Plasmodium falciparum malaria parasites that live in them and us, were the first in the world to develop (through over-prescribing and incomplete dosing) a resistance to chloroquine.

This is the drug which for decades has been humankind's main defence against malaria. I have suffered from malaria before, in Sudan, and I got through with little more than a debilitating fever, but that was the less deadly P vivax strain, which has yet to develop resistance to drugs.

Yet my fellow volunteers and I are not being foolhardy. We can be confident that whatever the parasite-load of the vectors filling their abdomens with our blood, we are unlikely to contribute to the worldwide toll of 2.7m malaria deaths each year (out of the at least 350m almost exclusively poor people who, according to a UN report of May 2005, sicken with the disease). In this fortunate village at least, there is a new and readily available treatment for P falciparum, a cocktail based on artemisinin, an extract from sweet wormwood. But it has to be administered swiftly.

All the locals understand, from the too recent and bitter experience of neighbours, that to contract this strain of malaria in Pailin and leave it to its own devices is to invite a rapid and painful death. After a week or more, we could expect fever, muscle pains and headaches, explains Bart Janssens, MSF's medical co-ordinator.

For any victims beyond the reach of artemisinin, however, diarrhoea, nausea and anaemia might develop. And finally, in a third to a half of all cases, there would be "severe complications", including brain damage, multiple organ failure and coma. "You could be dead in 10 days," he warns.

Our job that night, under the scrutiny of an entomologist, Dr Tho Sochantha, from the Centre for Malariology, is to help verify what the rapidly falling P falciparum prevalence and death toll has been suggesting: that MSF's volunteer-based, rapid-treatment programme for malaria in these forest-edge villages, where conditions are perfect for breeding mosquitoes, is "breaking the pathways of transmission" between female insects, parasites and humans.

My own contribution to the insect survey is only modest. I seem at first to be more attractive to ants than to mosquitoes. But finally, in the pitch-blackness, I learn to recognise the weightless, fussy probing of anopheles on my shins, and I begin to fill a test tube with my captives.

At the end of my shift, Dr Sochantha holds my tally up to his magnifier and, much to the delight of a crowd that has abandoned a blaring, battery-powered television set to watch a European make a meal of himself, declares my specimens to be "from a vector that normally prefers to feed on cows".

He would be happy to offer me a trapper's job at any time, though.

"You are a good hunter," he says. "Their abdomens are not bloated. You have captured them before they could feed." What I do not mention is that, though my leg has not been pierced, the back of my neck is already itchy and lumpy from a dozen uninterrupted bites.

Over the next few days, in the straw-roofed stilt villages of Treng Leu and Phnom Reang (or Tree Mountain), accessible to only the toughest off-road vehicles, the MSF mobile malaria team and its dignified and gentle Thai co-ordinator, Raden Srihawong, meet up with some of their 40 trained community volunteers to "mass-screen" every inhabitant for malaria. Their simple purpose is "to reduce malaria morbidity by finding more patients".

The villagers, in their mixture of traditional blouses, fake Nike or Puma T-shirts and an eclectic array of hats, are queuing in the mud at the temporary canvas-and-bamboo tents that MSF has erected for its medical inspection.

They are the strongest-looking buildings in the village. There is some crying from the children, who seem alarmed by the presence of so many strangers. But the adults are delighted to have so much free medical attention. These are some of the poorest communities in Asia. Not one has a school, electricity, sanitation, or water for washing or cooking, other than that provided by the heavens.

At this time of the year, the heavens have provided more than generously. A week of monsoons has turned the countryside into one great puddle. But for most of the year, the land is dry. "There are three harvests a year in Thailand," observes Bart Janssens. "But in Pailin there is probably only a half. Most farmers can produce enough food for six months, but then they have to catch insects and make soup from wild plants."

They have, too, to deal with the presence of unexploded ordnance. Amputees are a common sight, and signs warning "Danger! Mines!!" can be encountered every 20 yards, even on the narrowest of pathways. It is said that Pailin will not be entirely cleared for a further 80 years. Chea Doeurn, whose 18-year-old daughter, Chek Dany, is a malaria volunteer in the virtually unreachable village of Bor Thmei, has found many mines in his soya-bean fields. Three of their neighbours have died. Ten have lost limbs.

Their home is between "confirmed minefields" and "residual minefields" in the most mined area in the world, and is at the end of Route 10, where, as late as 1999 and eight years after the official ceasefire, the final skirmishes were exchanged between government troops and the last few Khmer Rouge insurgents. It is reached on foot by a newly cleared corridor that is barely a yard wide in places. Most of his neighbours are "retired" Khmer Rouge soldiers and may well have planted many of the mines themselves.

But they have no idea where. The landscape has changed so much. The district of Pailin, like much of Cambodia, was almost entirely forested 10 years ago. Now Pailin is mostly fields. The hardwoods have been felled and exported, mostly illegally. The thoughtless destruction of the forests (together with gem and petrol smuggling, bribery and unofficial taxes) has made government, police and army officials in Pailin, many of whom were the henchmen of Pol Pot's Brother Number Three, Ieng Sary, immensely rich.

Even the most modest of cafes in town are furnished with timber tables that would be worth a fortune in Europe. The villagers have not benefited, though, from the regime's sanctioned lawlessness.

"Cambodia exports its wood and imports its toothpicks," one of them comments drily. All the farmers have to show for the rape of the landscape are semi-fertile fields, rotting trees stumps, and mines.

Chea Doeurn, who is one of the many new immigrants who have come to these remote and risky borders because there is no shortage of smallholdings to squat and farm, recounts the story of a near neighbour who lost his right leg incrementally — ankle, knee, thigh — in three separate explosions. His daughter, Chek Dany, shudders to hear of it.

"Malaria is not our most frightening problem," she says, though she has suffered from the disease four times in the five years since her family arrived in Pailin, probably because as an outsider she has little immunity to it. On one occasion, she was so ill that she reached what she calls "a staring coma". Now, as MSF's "village face", she offers malaria advice, diagnosis and treatment to most of the 370 inhabitants within her district. But she has yet to dare to visit some of her nearest neighbours because the ground between them is so full of landmines.

"I think a lot about the danger when I am working," says Chea Doeurn, sitting on the decking of his house-cum-shop where he sells vegetables and glasses of home-brewed rice spirit which — as I can verify — must have caused some "staring comas" of its own, "but I have to farm the fields because I am a poor man. I am more frightened of hunger than I am of the mines."

The good news is that thanks to the work of MSF and its village volunteers, at least the killer form of malaria is retreating from their lives. The 2003 prevalence survey found 7.8% of these rural populations positive for P falciparum (and showed that 5% of the countless millions of mosquitoes were carriers). In 2004 the "positive" figure was down to 3.7%. Already the 2005 surveys are indicating a heartening prevalence of less than 1.4%. Certainly, in the two communities of the mass screening, those few villagers reporting malaria-like symptoms on this occasion and judged symptomatic when examined physically are few and far between.

Most people are marked by ink pen on their arms with an ID number and a blue A for Asymptomatic (clear of disease). Of this overwhelming majority, only those pregnant women needing iron tablets, or children who require deworming, or anyone with a rash, an infection or an abrasion is sent to Raden Srihawong for treatment. But the few who are sporting the red S of Symptomatic proceed to another set of blood tests and Parachecks. Among them is No 191, an 11-year-old boy called Phan Mol who suffers from malnutrition.

He weighs a little over three stone, less than a healthy five-year-old. He is stoical when Raden examines him. But Phan Mol, like most of those marked S, is lucky. He has the less dangerous P vivax. Raden dispenses the chloroquine there and then.

No 67, Uong Virakk, is less fortunate. As the red PF( ) mark on his forearm indicates, he has P falciparum and is very ill. He lies on his back under yet another battery-driven television set. He is nauseous, shivering, struggling to breathe. No 82, a three-year-old girl, is PF( ) also. So are numbers 107 (a pregnant 22-year-old), 149 (a young farmer), and 26 (an elderly woman, recently widowed, who has come to the village to live with her daughter and has not been exposed to malaria before). Out of almost 600 villagers tested, only these five have been in any danger from fatal malaria. In three days' time, after their courses of artemisinin, they will be cured.

Here, for once, is evidence of a medical success story. Indeed, I myself have cause to be grateful to MSF and its efficient strategies. Two weeks after my exposure to the mosquitoes of Pailin and back home in Birmingham, I begin to sweat and shiver. Every muscle in my body aches. I prick my fingertip and carry out the simple Paracheck that all the village volunteers offer to their symptomatic patients. I show negative for P falciparum. It's vivax again. Raden Srihawong has sent me home with all the treatments and I am well again within a few days.

Sadly, the medical success story of Médecins Sans Frontières and Plasmodium falciparum does not extend to other health issues in Pailin. Elsewhere in the district it is mostly a case of Frontières Sans Médecins. This is a country ravaged by war and by one of the 20th century's most brutal regimes. Between 1970 and 1978, about 2m citizens — one third of the population — were murdered. Cambodia has not recovered yet. Its public health provisions, nonexistent under Pol Pot and scarcely any better during the 10 years of the Vietnamese occupation, are still poor.

The government spends less than £1.75 a year on health care for each of its citizens. Infant mortality rates, according to the 1998 census, are almost 1 in 10. Life expectancy is 57.

Most Cambodians die from conditions that are simple and cheap to cure or contain. There are, as yet, few foreigners in Pailin district. There are certainly no tourists, except for those who come across the Thai border every night for the gambling and the "karaoke bars" (or brothels). Any other outsiders that do plan on spending any time in what has been called Cambodia's "badlands" and its "Wild West" should not expect to go away entirely unscathed. Here it is not easy to stay well.

The US Department of Health advises travellers to Cambodia to protect themselves with seven vaccinations. It warns against almost 30 health hazards, from avian influenza — via plague, polio, rabies and (still) Sars — to typhoid. The guidebooks advise travellers to refuse salads, peeled fruits, meat products and anything but bottled drinks. They should evade the endemic ticks, the chiggers and the many parasitic worms by never walking in water or in bare feet. They are exhorted not to venture from a surfaced road into the landmined countryside. "Even for the call of nature: your limbs are more important than your modesty."

But any traveller who completes the four-hour drive from Battambang into Pailin district will realise at once that those cautious (and expensive) lifestyles recommended by guidebooks are impossible and unsustainable even for tourists. For locals, illness is almost unavoidable, as is medical neglect, even if they are reckless enough to enter one of the dozen or so single-storey buildings in the waste- and weed-filled compound that is the disgrace of Pailin hospital, a place of Crimean inadequacy.

The director is ex-Khmer Rouge and is not medically trained. Like many of his 70 or so colleagues (of whom, reportedly, only 20 show up at work each day), he was given his job as a sinecure by powerful old comrades, men like the district's current (and ex-Khmer Rouge) military commander, Brakk Sakhorn, or the provincial governor, Y Chhean, who learnt his administrative skills in the forests as a Pol Pot stalwart.

Certainly, the director does not seem much interested in the sick. He manages to tour the filthy and almost empty wards without even looking at any of his patients. He seems bored, disengaged, uninquisitive. A used condom on the floor goes unnoticed. So does a young woman in the last hours of her life. The one male nurse will not help her either, though only five of the 20 slatted beds in his ward are in use. She is a sex worker and she is in the final stages of Aids. She is spider-thin. Only her eyes — disoriented, exhausted and terrified — are not reduced in size.

The only care she gets is a pile of damp sawdust under her bed, to soak up her faeces and urine. She has been offered no medicines, though anti-retroviral drugs — available in some areas of Cambodia — might have saved her life. It is not until five volunteers from Family Health International arrive that she is afforded the dignity of a change of clothes, a wash and some food (which she cannot swallow).

These volunteers, most of whom are sex workers themselves and some of whom are HIV-positive, are keen to show their contempt for the staff, for their laziness, cowardice and heartlessness.

Mam Savry's husband had the good sense to stay away from this local Pailin hospital, despite his illness. He, like many Cambodians, had been frequently to prostitutes such as the dying woman and her volunteer carers. In 1996 he'd caught venereal disease and passed it on to Mam, his second wife, a small and spirited woman with a mouthful of gold teeth. They paid for a private cure but, she says, "our marriage was already damaged because he had brought violations to the family".

Then, two years later, she says, "He began behaving strangely. He was feverish at my side in bed. It felt as if steam was coming off his body."

He was diagnosed as HIV-positive and so, when he was 43 and seriously ill, he deserted Mam Savry to return to his birth village close to Pailin town where his family and his first wife still lived. He took no medicines, except those offered by the khruu khmer, traditional healers. It took him a year to die, a year of fevers and skin diseases, chest infections and unrelieved pain.

Mam Savry determined to make a new life for herself with relatives in Siem Reap, Cambodia's richest, most modernised and most fashionable city — the one most favoured by tourists.

The ancient temples of Angkor are only a 20-minute ride away by tuk-tuk. That decision saved her life. She had feared that her husband might have passed on his HIV, but she had never found the courage to take a blood test.

"I did everything I could to make myself happy," she says, "but nothing could put an end to my fear."

Then she fell victim to a long series of infections and to intermittent diarrhoea. Both she and her relatives recognised the symptoms.

Her weight fell from her usual 42kg to 29kg. Her family, her neighbours and the community treated her as if she were a contagious embarrassment: "For example, my porridge was tipped into a plastic bag for me to eat with my fingers and not put on a dish to eat with a spoon like everyone else." But help was at hand. It was 2002 and Médecins Sans Frontières had just opened its Chronic Diseases Clinic in the grounds of the Siem Reap Provincial Hospital. "My friend told me there was a treatment which could cheat death and give me a long life, and so I found my courage and I turned up at their door."

Mam Savry was one of MSF's first HIV patients in Siem Reap to receive the anti-retroviral drugs that so effectively aid the immune system to fight opportunistic diseases, such as tuberculosis, pneumonia, meningitis, and a host of piggyback viruses. It is these, and not the HIV status, that transform the condition into the full-blown Aids that so rapidly killed her husband. If Mam Savry had fallen ill before 2002 in Siem Reap, where the HIV prevalence of more than 2% is currently the highest in Cambodia, she too would have been left to die.

Nowadays, after three years of a therapy that, since the introduction of generic drugs, has cut the treatment costs from £5,700 a year to just £80, she is "feeling strong" and "full of hope". It is hard to imagine, as she bustles around the MSF clinic as a volunteer counsellor, that Mam Savry could possibly be suffering from the same condition as the dying sex worker in Pailin.

The same is true of Sim Chorb, a 33-year-old rice farmer from Prey Thmey village, who has been HIV-positive for 15 years, after one too many visits to a prostitute. More than a quarter of all sex workers have the disease. Sim Chorb's friend died without treatment, but once Sim became sick with tuberculosis and diarrhoea, he was brave enough to take a blood test.

His CD4 count (the immune-defence lymphocyte most targeted by HIV) was as low as 30. A count of around 1,000 is normal. Now he swallows six tablets a day, taken from a batch supplied by MSF. He cannot forget the disease in his body (and claims not to visit brothels nowadays) but he is fit and well. So, too, is Lim Sovaan, a 52-year-old grandmother who looks 15 years younger.

She lives in the outskirts of Siem Reap — or Thailand Defeated, as it means in Khmer — where she makes a meagre living recycling tins and bottles. It is a 20-minute wade through warm, sewage-rich water to reach her flooded hut. She once had a less modest house, but when her husband died of Aids she had to sell it to settle his medical debts.

Her husband had turned to the local khruu khmer for help. They gave him expensive herbs to boil.

"We did not understand the illness then," she says. "Some people thought Aids was spread by mosquitoes. Others said that coffee was a cure."

When her own health deteriorated, however, she joined the more than 900 — out of 1,430 positive patients — currently being treated with anti-retroviral drugs, and supported by home visits, health advice and dietary supplements.

"This clinic is a place of hope," says MSF's project co-ordinator, Dr Sophie Duterme, as she walks towards the clean, fan-cooled ward under her control where most HIV patients are suffering from opportunistic diseases or the rare, severe side effects to treatment, including toxic liver damage and neurological failures. "Even though we have patients with a desperate condition, this is not a place of desperation. There is a problem, though. Until last year, up to two-thirds of our patients were coming from outside the province. Now we have been forced to limit the number of new patients to 50 a month — and they must come from this province." So Pailin people are refused? "Yes, that is so. Regrettably."

Siem Reap provincial hospital, with its single-storey blocks and its shaded open ground, is the most peaceful section in the busy centre of town, close to the Old Market quarter. But not for long. It has to move elsewhere. The developers have their eyes on it, as they have their eyes on almost every acre of land where they might throw up a cheap hotel, Florida-style. Cheap to build, that is, and cheap to run. Staff wages, for those applicants who pass an HIV test, are low.

But the room rates will be "international" and expressed in US dollars, now the town's semi-official currency, and not in the local riels that even beggars will refuse. The nearby temples of Angkor, built so lovingly a millennium ago, are occasioning a rushed and thoughtless epidemic of bed fever, not beds for the sick, of course, but for the target number of 15m visitors a year. In 2004 there were just 1m tourists for the whole of Cambodia. Already, the road out to the airport has become a Via Dollarosa, a dispiriting, vulgar strip where fly-in, fly-out trippers, mostly from Japan, Taiwan and South Korea, are quarantined and anaesthetised against the real Cambodia.

Those who do bother to wander in the Old Market quarter will discover not only some of the finest food in Asia and the usual national charm and courtesy, but also legions of amputees, many from Pailin district, tuk-tuk drivers unprepared to take no for an answer, gangs of waifs with their baby siblings as begging accessories, men with anything and everything to sell. All are seagulls to the crust. And all are so skilled at importuning, bullying and wheedling any visitor who does not simply walk away that in the space of an afternoon I had handed over $8, more than a Cambodian doctor would earn in a week.

It seemed to me that I was targeted because, as a male, middle-aged European walking alone, I fitted the profile of the sex tourist — of which, on the evidence of the mixed-national, mixed-age, mixed-sized couples on the street, there are many. So, in a country where genuine massages were once part of the culture, I was summoned from the doors of parlours so blatantly "untraditional" that even the words "pedicure" and "reflexology" were laden with a new, grubby meaning. I was offered "brides" by shopkeepers.

And most alarmingly (and within plain sight of a street hoarding that declared in English: "Abuse a child in our country; go to prison in yours"), I was tagged by a suggestive nine-year-old girl who, when I would not let her take my hand, refused to go away until I offered her a dollar. "You too old anyway," she said.

Siem Reap, then, is a town that's being ruined by its closeness to some ruins. Too soon it will be intolerable, a victim of the Revolution of Rising Expectations that seems to attach a drawback to every advantage it offers. Its new, though selective, wealth and modernity seem like an impediment to progress rather than evidence of it. Fortunes will be made, of course, by the already rich and powerful.

But there is a maturing poverty of spirit in Siem Reap, and although the foreign visitor will be met with hopeful looks by almost everyone, the hope is only that the visitor will spend. In rural Pailin, though, not once was I requested to give anything. The only demand on me was that I should spend time beneath their roof to talk, and risk, perhaps, a glass of their rice spirit and some mosquito bites. That remote and backward Cambodia, for all its deprivations and its inequalities, deserves a better future than the one being mapped out in the shadows of Angkor. This is the paradox of progress.

Here, then, are two Cambodias that offer a health-care lottery for its citizens — or at least those who cannot afford to pay for private care in Thailand. One is tended by non-governmental organisations such as Médecins Sans Frontières. Those few Cambodians fortunate enough to live within MSF's specialist orbits — for malaria in Pailin, for Aids in Siem Reap and four other clinics — can expect treatment as efficient and effective as any offered by hospitals in Europe. The other is, at best, less reliable.

Its harshest critics would say that, in a country where each year the same amount — £285m — is received as official aid as is lost from the economy to corruption, the future is not promising.

"What's happening in Cambodia is a mixture of a virtually unregulated private sector, a very badly educated population that doesn't have the capacity to distinguish between good and bad, and a government that is so lowly motivated and unambitious in health care that even when it does spend money it spends it ineffectively," says Bart Janssens, as he contemplates what will happen when MSF withdraws, as it must within the next few years, from its projects in Pailin and Siem Reap.

"The government health service in Pailin has not been able to deal with malaria on its own. So how can we trust their ability to deliver this complicated long-term Aids treatment on their own?" There is silence, as nobody can find the energy or evidence to argue that, because of government promises to expand and improve, this judgment is unfair. It is a silence filled with fevers, chills, staring comas, diseases quick to seize their opportunity, and piles of sawdust under unattended beds.