By Nick Triggle
BBC News health reporter
The global health industry is worth $3trn
Rumours began circulating in the 1990s about corruption in the Thai health system.
By 1998 a body representing rural doctors had blown the whistle, claiming officials in the country's health ministry were involved in a scam over the procurement of drugs and medical supplies.
The Rural Doctors Forum said its members had been told to favour certain companies, sometimes buying goods at three times the going rate.
An official investigation was eventually launched and found evidence of corruption.
Ministerial resignations and civil servant sackings followed and the public health minister at the time ended up in jail after being found guilty of accepting bribes from drug companies.
This is just one example of the corruption within the $3 trillion global health industry highlighted in a report by Transparency International.
The case may not be unique, but it does illustrate why the sector is so vulnerable to corruption.
What marks the health sector out is that there are large sums of money - often public - washing around in complex systems involving a variety of players from doctors and patients to drug firms and government officials, all of whom have competing interests.
In Thailand, the forum said health officials were able to wrestle control of procurement from individual hospitals to take advantage of the vast sums of money linked to the drugs industry.
And it is this principal-agent relationship, which exists throughout health systems the world over, which seems to be at the crux of the problem.
Alan Maynard, professor of health economics at York University, said: "The pharmaceutical industry is worth a lot of money, so you do hear of doctors and officials being whisked off on ski trips and the benefits of drugs being pushed.
"There is all sort of chicanery and practices that are less than ethical. Even in countries where regulations are in place, it can be hard to police."
But it is not just drug companies and officials which are at the centre of corruption.
Professor Maynard also says in many countries where pay is low, doctors can abuse the system.
"You hear all sorts of reports about doctors accepting, or asking for, bribes for treatment."
Part of the problem, the report by Transparency International said, is that there is an imbalance of information.
Doctors known more about treatment than patients and if their income depends on what therapies they use, the temptation is clear.
Such conflicts of interests led aid agency Medicins Sans Frontieres to collaborate with Cambodian authorities to top up salaries for health workers to stamp out unethical practices.
The indications are that the project, which has been running in the Reap province since 2001, has been successful.
John Williams, director of ethics at the World Medical Association, admitted corruption can be a problem.
"Physicians are human beings and, like everyone else, are subject to the temptation to put their own interests above those of others.
"As self-regulating professionals they have less oversight than many other individuals, and consequently more opportunity to conceal unethical behaviour."
But in some respects, corruption may be unavoidable because of the very natures of the sector, as the Transparency International report acknowledges.
The report said not knowing who will fall ill, when it will strike and how bad it will become, means the health sector is often inefficient and prone to corruption.
And if an emergency situation develops during which various oversight procedures are overlooked, the risk is even greater, it added.