By Jill McGivering
BBC World Service's Assignment programme
Preah Vihear province - in northern Cambodia, close to the Thai border - is a lush jungle region that used to be a stronghold of the Khmer Rouge; a remote, underdeveloped province. It also has a high rate of malaria.
Cambodia's hospitals have an influx as the malaria season starts
In the small town hospital is Jai Prok, a thin 63-year-old. When he first became ill he tried to treat himself. He bought malaria medicine from a hawker who came to his village but it simply did not work.
And such an experience is not unusual, according to Solang, the man in charge of the hospital's medicines.
"There's no way patients can tell the difference between fake drugs and the real thing," he says.
"Even some doctors can't tell. They all look the same."
It is hard to get an accurate picture of how many anti-malarial drugs are fake.
A group of malaria specialists, part of a collaborative project involving Oxford University, Bangkok's Mahidol University and the Wellcome Trust, have carried out small-scale surveys in Cambodia, Laos, Vietnam and Burma.
What is certain, says professor Nick Day - a leading member of the team in Bangkok - is that fake drugs are a massive problem in South East Asia.
"In some areas 30-50%, or even more than 50%, of drugs you buy randomly from pharmacies are actually fake," he says.
"They contain no active anti-malarial ingredient."
The composition of these fakes is wide-ranging. Some are totally inert substances like starch. Others contain cheap drugs like paracetamol.
Others do contain small quantities of the artesunate-type anti-malarial drugs - probably only to fool the testers - but ironically it is these that constitute a major problem in themselves.
Global resistance to the last generation of malaria drugs first emerged in this region. Now scientists are worried that these small amounts could expose parasites to a level of the drug that is not enough to kill them, but could select for resistant parasites.
The artemisinin class of drugs, of which artesunate is one, is the most powerful and effective in treating malaria - and the impact of parasites developing resistance to them would be disastrous, says professor Day.
Particularly in Africa, which sees about 90% of the world's malaria cases, it would be "a public health catastrophe," he adds.
And he describes the criminals responsible for this growing trade as murderers.
They are sophisticated enough to know exactly what they are doing - and the impact their fake drugs have on the poor.
The criminal network involved is well-organised, on an industrial scale, and trans-national. Interpol is now involved in trying to track them down.
One of their senior investigators, John Newton, says that in South-East Asia, the common denominator is that the criminals are ethnic Chinese.
They may be from mainland China, Malaysia or Burma, but because they know each other they are very difficult to infiltrate.
The fake drugs trade is generating "significant profits", he adds.
Anti-malarial drugs are essential in many parts of the developing world
"The fact it's industrialised shows a lot of investment has gone into it. It's a significant problem and it's growing."
China's actual role is unclear. It is a major legitimate manufacturer of artesunates; the artemisinin family of drugs came from China in the first place, and they're derived from sweet wormwood, whose properties have been known to traditional Chinese medicine for more than 2,000 years.
But China is also, in general, fighting counterfeiters. So could the same country which gave these new life-saving drugs to the world also be playing a part in destroying them?
For example, fakes of one of the brands owned by Holleypharm, which makes five million doses of the anti-malarial drugs a year at its factory in Chongqing in central China, have already been found in Africa.
Company president Charles Lu says China needs to strengthen its regulations for pharmaceuticals made in China for export overseas.
I told him that although many people I had spoken to had been cautious about publicly accusing China publicly of being a main source of these fakes, in private they say it is one of the few countries that has the technology to produce such sophisticated counterfeits.
"I think maybe partly it's true," said Mr Lu.
"If people say all the counterfeit is coming from China, I don't think that's true."
He thinks private businessmen are behind the trade.
The Chinese are now more involved in tracking fake drugs
"They are very slippery - they can decide what they do today and maybe they change tomorrow," he adds.
"I don't think the big pharmaceutical companies can do that. They are also controlled and inspected by the Chinese authorities."
The World Health Organization's representative in China, Dr Henk Bekedam, says China has become more willing in the last few years to address this problem.
"When I started here in 2002, the government was initially very reluctant to talk about the issue of fake drugs," he says.
"They said they're not part of it. Now I think we have moved on. China is accepting that fake drugs are a global problem. They are part of the world, they are part of the problem - but they are also part of the solution."
There is more China could do. So far, for example, China has declined to join a recently established regional alert system through which professionals can rapidly inform each other about drugs they're suspicious about without. The WHO hopes China will sign up soon.
The artesunate family of drugs is, quite simply, the only real weapon mankind has left in the fight against one of its most deadly diseases. Unless drug piracy is defeated, even that last weapon could be taken away.
Jill McGivering's full report can be heard on BBC World Service's Assignment programme on Thursday 7 June at 0806 GMT.