In the days before "do-gooders" became a term of disapproval, doctors and scientists were held in absolute public esteem. They did the most good; they were working altruistically to benefit the human race - to cure illness, prevent disease and create a safer, healthier future for us all. That, at least, used to be the popular perception. Plainly, the image has become more than a little tarnished. Even before the scandal of the deaths of babies at Bristol Royal Infirmary, enough instances of incompetence and negligence had emerged to provoke widespread public scepticism about the professions. These days, scientists are more likely to find themselves occupying the lower rungs on the ladder of public trust, alongside estate agents and, well, journalists.

It is also increasingly understood that scientific research is now hardly ever conducted in a spirit of disinterested inquiry. Usually, it is funded by global companies whose concerns are anything but disinterested. Even when research is financed by government agencies, those, too, will want to call the tune. According to a survey carried out last year by the scientific body, the Institute of Professionals, Managers and Specialists, one in three scientists working for government quangos or newly privatised laboratories has been asked to adjust conclusions to suit the sponsor.

Leaving aside the implication of this for forensic science, it is evident that most scientific inquiry today is dictated not by the thirst for knowledge but by the thirst for profits. Even so, the full extent of the betrayal of the public interest has yet to be appreciated. Internationally, the scientific community has been responsible for serious errors, which have then been covered up with devastating consequences for public health. There was no proper treatment available for victims, as their condition was undiagnosed; and the same mistakes were repeated elsewhere.

Twenty years ago, the Spanish "cooking oil" disaster began as a mystery illness. Years later, the toll was put at more than 1,000 deaths and more than 25,000 seriously injured, many of whom were permanently disabled. It was the most devastating food poisoning in modern European history.

The disaster is historically important not just because of its scale and the number of victims. It was the prototype contemporary scientific fraud. It marked the first time that multinational interests successfully contrived a major cover-up in international science. For the one thing that is certain about the Spanish "cooking oil" disaster is that it had nothing to do with cooking oil.

The epidemic is officially deemed to have started on May 1 1981, when an eight-year-old boy, Jaime Vaquero Garcia, suddenly fell ill and died in his mother's arms on the way to La Paz children's hospital in Madrid. Learning that his five brothers and sisters were also ill, doctors had them all brought in and put one of the girls into intensive care. The other four children were transferred to the Hospital del Rey, Madrid's prestigious clinic for infectious diseases, where doctors began treating them for "atypical pneumonia".

When the director, Dr Antonio Muro y Fernandez-Cavada, arrived at work the following morning, he was alarmed to be told that these new patients were being treated for pneumonia. He gave his staff a dressing-down; it was out of the question medically for six members of a family to be suffering the same symptoms of pneumonia at the same time.

The Vaquero family proved merely the first of many. It seemed to be mainly women and children who were affected. The initial symptoms were flu-like: fever and breathing difficulties, vomiting and nausea, although patients soon developed a pulmonary oedema (the build-up of fluid in the lungs), skin rashes and muscle pain. The epidemic was national news.

After a few days, Muro told the media that he believed it was due to food poisoning, adding that the foodstuff was marketed "via an alternative route". He was certain of this because the casualties were all coming from the apartment blocks of the communities and towns surrounding the capital; almost no one from Madrid itself appeared to be affected.

Muro brought together relatives of those afflicted with the mystery illness and told them to work out exactly what the victims may have eaten that they, the unaffected family members, may not have eaten. In half an hour, they had an answer: salads.

On May 12, Dr Angel Peralta, the head of the endocrinology department at La Paz hospital, pointed out in a newspaper article that the symptoms of the illness were best explained by "poisoning by organo-phosphates". The following day, he received a telephone call from the health ministry, ordering him to say nothing about the epidemic, and certainly nothing about organo-phosphorous poisoning.

That same day, Muro invited health ministry officials to the Hospital del Rey. He produced maps of the localities, showing where the patients lived. He believed that the contaminated foodstuff was being sold at the local weekly street markets, the mercadillos, which set up in different towns on different days. On this basis, he predicted where the illness would strike next. He was proved right, but this was scant consolation for the fact that he was suddenly informed that he was relieved of his duties as hospital director, with immediate effect. His dismissal at least enabled him to carry out his own first-hand investigations. He patrolled the mercadillos and noticed the popularity and cheapness of large, unlabelled plastic containers of cooking oil. Immediately, he and his colleagues, one of whom was Dr Vicente Granero More, went to the houses of affected families and removed the containers of oil that they had been using when they fell ill. They carefully labelled them, sent samples of each to the government's main laboratory at Majadahonda, just outside Madrid, and awaited the results.

Most medical personnel were simply trying to tend the sick and dying - a difficult enough task in optimum conditions, but one made almost impossible because doctors, not knowing the cause of the illness, had no idea how to treat patients. Further, as the illness reached its chronic stage, the symptoms became more severe, and included weight loss, myalgia, alopecia (hair loss), muscle atrophy and limb deformity.

At all administrative levels, there was bewilderment and anxiety. Spain was then still a fledgling democracy; the dictator, General Franco, had died as recently as 1975. In February 1981, only three months prior to the outbreak, a lieutenant-colonel, Antonio Tejero, had held MPs in the cortes (parliament) at gunpoint in a botched attempt to restore army rule. More than a month after the epidemic first struck, most of those in power had no strategy other than to hope something would turn up. Finally, it did. Dr Juan Tabuenca Oliver, director of the Hospital Infantil de Niño Jesus, told the government that he'd found the cause of the epidemic. He'd asked 210 of the children in his care, and they'd all consumed cooking oil.

A fter, it seems, some initial hesitation, the government accepted his theory. On June 10, an official announcement was made on late-night television, informing the public that the epidemic was caused by contaminated cooking oil. Almost immediately, the panic subsided. The hospitals remained full of victims, but new admissions dropped sharply. The situation seemed, at least, under control.

Yet the government's announcement had been watched with stunned disbelief by Muro and his colleagues. Only the previous day, on June 9, they had obtained the results of the tests on their own, precise oil samples. These showed that, although none was the pure olive oil that the vendor had no doubt claimed it to be, almost all the oils had different constituents. Such a variety of oils obviously could not account for one specific illness.

The cooking oil theory was superficially persuasive. To protect its native olive oil industry, the Spanish government tried to prevent imports of the much cheaper rapeseed oil, then being put to widespread use throughout the European Community (which Spain did not join until 1986). Imports of rapeseed oil were allowed only for industrial use; the oil first had to be made inedible through the addition of aniline.

Streetwise entrepreneurs simply imported the cheaper oil anyway. The more scrupulous among them then removed the aniline; the others didn't bother. The illness was therefore attributed to aniline poisoning. It became colloquially known as la colza (which is Spanish for "rapeseed"). A number of the more high-profile oil merchants were arrested.

Three weeks after the television announcement, the health ministry allowed families to hand in their supposedly contaminated oil and replaced it with pure olive oil. This belated exchange programme was hopelessly mishandled, with few authentic records kept of who was exchanging what or (and this should have been the key point) whether the oil came from affected or unaffected households. As olive oil was guaranteed in return, many people simply handed in any oil they could find, even motor oil. Most of the oil that supposedly caused the epidemic was never available for subsequent scientific analysis. The instinctive reaction of most families, upon hearing that it was to blame for the illness, had simply been to throw it away.

In order to demonstrate that the oil had caused the illness, government scientists needed to be able to show, for example, that families who had bought the oil were affected, whereas those who hadn't were not; that the aniline in the oil was indeed poisonous and that the victims were suffering from aniline poisoning; and, bearing in mind that such commercial cooking oil fraud had been widespread for years, just what had changed in the manufacturing process to cause the oil suddenly to become so poisonous. To this day, none of these basic conditions has been met.

In 1983, however, an international conference was convened in Madrid under the auspices of the World Health Organisation (WHO). Despite the reservations of many scientists present, the epidemic was then officially named toxic oil syndrome (TOS). In 1985, the opinion of the internationally respected British epidemiologist Sir Richard Doll was sought. He was cautious, saying, "If it could be shown that even one person who developed the disease could not have had exposure to [the oil], that would provide good grounds for exculpating the oil altogether."

The trial of the oil merchants began in March 1987. Four months later, Doll, just before giving his evidence, announced that, on the basis of fresh epidemiological reports given to him, he now believed that the oil was the cause of the outbreak.

At the end of the two-year trial in 1989, the judges themselves stressed that the toxin in the oil was "still unknown". This somewhat fundamental difficulty did not prevent them from handing down long prison terms to the oil merchants, who were convicted, in effect, of causing the epidemic.

After years of one-track media reports, the notion of the "cooking oil" epidemic was firmly lodged in the public consciousness. It was unquestioned fact. No one doubted the official scientific conclusions, especially as they were accepted by the WHO.

After the 1983 Madrid conference, when there was still widespread unease with the oil theory, the Spanish government recruited some of the country's leading epidemiologists to head a fresh commission of inquiry. Among those chosen were Dr Javier Martinez Ruiz and Dr Maria Clavera Ortiz, a husband-and-wife team from Barcelona. "We absolutely believed the oil was to blame," they said. "We thought the only problem was that the information was disorganised and the research inadequate."

So they set about a rigorous examination of the official information. The results shocked them. Martinez looked at the pattern of admissions to hospitals and realised that the epidemic had peaked at the end of May. The incidence curve went down at least 10 days before the government's June 10 broadcast, and about a month before the withdrawal of the oil. In fact, the announcement that oil was to blame had had no effect on the course of the epidemic.

Meanwhile, his wife had examined the patterns of distribution of the suspect oil, which had come across the border from France. She realised that vast quantities of the oil were sold in regions (notably Catalonia) where there had not been a single case of illness. And they subsequently learned that the government was already fully aware of this. At the time of the epidemic, the government had created a new post of secretary of state for consumer affairs at cabinet level. Chosen for this appointment was a rising lawyer and economist, Enrique Martinez de Genique.

Genique himself had drawn up maps of the distribution of the oil and the pattern of illness. He realised that there was no correlation between the two and, accordingly, that the oil was not the cause of the epidemic. After presenting his findings to the health ministry, he was sacked from his government post, and soon decided to retire from politics altogether. He emphasised that he had never regretted what he did: "I had very grave doubts [about the government's stance on the epidemic] and I was morally and ethically obliged to voice them."

Martinez and Clavera, too, were fired. As this did not entirely prevent the possibility of the commission reaching inconvenient conclusions, it was soon closed down altogether.

The powerful, indeed irrefutable, evidence that the suspect oil was sold throughout parts of Spain where not a single case of illness resulted could be coupled with equally clear evidence of the converse: of people who could not have been exposed to the oil falling victim to the epidemic.

While making a television documentary, I saw many families who had suffered illness yet were adamant that they had never purchased the oil. One woman used only supplies from the olive groves of her relatives in Andalusia, yet she was seriously disabled by the illness. Perhaps the best authenticated example was the case of Maria Concepcion Navarro, a young lawyer in Madrid who fell ill, became progressively worse and died in August 1982. Her symptoms were exactly the same as those of other fatalities of la colza and she was put on the official roll of TOS victims - despite the fact that her husband, also a lawyer, stressed that they had only ever used the most reputable cooking oils. Then the authorities belatedly noticed another significant contradiction. Maria Concepcion had actually been hospitalised from November 1979, 18 months before the start of la colza. She didn't fit the official theory; consequently, her name was struck off the list of victims.

On a broader scale, this was how the statistics of the epidemic were compiled. If victims - afectados - or their families agreed that they had used the oil, their names were added to the official list; if they asserted that they had never had the oil, their names were excluded. However, the health ministry had made it known that only those whose names appeared on the official list would qualify for government compensation, so there was a clear incentive for afectados to say that they had used the oil. Developments like this artificially buttressed the government's position and made it almost impossible to produce an accurate assessment of the epidemic.

T hrough all the obfuscation, one man had simply ignored the official lines of inquiry and spent months pursuing his own. Having eliminated the cooking oil, Muro and his colleagues turned their attention to other salad products. Speaking to market stallholders, lorry drivers and around 4,000-5,000 affected families, they concluded that, without any doubt, the contaminated foodstuff was tomatoes, and it was the pesticides on them that were responsible for the epidemic. The organo-phosphorous chemicals would indeed cause the range of symptoms observed by clinicians.

The tomatoes, they established, had come from Almeria, in the south-east corner of Spain. Once a desert area, this was not fit for crop-growing until the discovery of underground water in the 1970s helped to transform it into an agricultural success story. Fruit and vegetables were forced into rapid growth under long tunnels of plastic sheeting. Some farmers got three, or even four, crops a year.

This agricultural boom was made possible only through the application of copious quantities of chemicals: nutrients, fertilisers and pesticides. Although exactly what happened may never be known, it is likely that one farmer had used the chemicals too liberally, or had harvested the crop too quickly after applying them. Neither would have been surprising. Some of the farmers were illiterate and would have had difficulty with the instructions for use on the containers of chemicals.

Muro had many supporters but, as the official view became more and more entrenched, so he was marginalised as the one dissident voice. In 1985, he died suddenly of a mysterious illness. His wife perceived the whole saga as an unmitigated family tragedy.

It was Muro and his team who had done the on-the-ground epidemiology in the immediate wake of the outbreak. What, then, of the epidemiology that the WHO in 1992 was boldly to describe as "comprehensive and exacting epidemiological studies, subjected to critical independent assessment"?

Muro's work was first-hand. But trying to assess the accuracy and validity of the official epidemiology was not easy. The FIS - the government agency responsible for toxic oil syndrome - refused to release details of the fieldwork carried out or any background information. However, the families described in the reports were given code numbers and these could be matched against the official list of victims which then became part of the trial documentation. Eventually we identified the families supposedly interviewed for the key epidemiological reports and went to see them.

From these first-hand inquiries, we established that there was not a single case in which the family's history corresponded with what was written in the epidemiological reports. Sometimes the differences were slight; sometimes the reports bore no relation to what had actually happened. In one sense, this was not surprising; while some families did recall having been interviewed by officials at the time, others insisted that they were never questioned at all. The principal scientific premise - that evidence should be gathered and, on that basis, a conclusion reached - appeared to have been reversed: a conclusion had been reached, and then the evidence manipulated in order to support that conclusion.

The original study on which the oil theory is founded, by Dr Juan Tabuenca Oliver, was published in the Lancet; yet it appears less than rigorous. At the outset, he claimed that all 210 of the children in his care had taken the oil. The next time that reference was made to this study, the number of children in his care was given as 60. Two years later, it had shot up again, to 345. Today, the figure is put at 62.

Moreover, his claim that all of the children had consumed the oil was disputed at the trial. Pilar Pans Gonzalez, the mother of one of his patients, was asked if her son had had the oil. She replied that he had not. Asked how she could explain this discrepancy (with the supposed 100% finding), she replied, "That is their problem, something they have invented."

There are three specific epidemiological reports on which the oil theory now rests. Two of these are particularly astonishing. The first concerns three cases of illness in two families in Seville. These three became ill, according to the official analysis, because the heads of the families worked at a refinery where some of the suspect oil had supposedly been refined, and took some for use at home.

There are a number of glaring problems with this report. Most importantly, one of the families, on hearing the government's announcement about the illness, had taken their own oil to be analysed at the local Instituto de la Grasa, which happened to be one of the country's most renowned laboratories. The records of this analysis are still available; the oil was not rapeseed at all, it was olive oil.

If the theory was correct, one might have expected other refinery workers to fall ill; no one did. However, there were originally, according to the government's own records, 83 cases of la colza in Seville. The other 80 vanished from the official records, presumably because they couldn't possibly fit the oil theory; after all, the suspect oil had never been sold in Andalusia, where authentic olive oil is in such plentiful supply.

Even more amazing was the study concerning a convent outside Madrid. According to this, 42 out of 43 nuns fell ill after using the oil, while visitors whose food was prepared in a different oil did not fall ill. From an official perspective, the beauty of this epidemiology was not just that it provided game, set and match for the oil theory, but that no one could afterwards check the veracity of the paper. This was a closed convent. The nuns had no routine contact with members of the public, and they certainly didn't talk to the media. In the event, senior nuns from the convent did give evidence at the trial. Their testimony flatly contradicted what was written in the convent report. Of course, all the food was prepared in the same way and cooked in the same oil. In fact, only very few nuns (about eight or nine) suffered any illness. The epidemiological report was a fabrication.

Nor was the oil theory underpinned by any laboratory science. In the years since the 1981 outbreak, the suspect oils have been analysed in leading laboratories throughout the world. No chemical, or contaminant, that would account for the symptoms observed in the afectados has ever been found. Aniline - which was blamed for the epidemic - is poisonous only in much greater quantities than were present in the oil and, in any case, the symptoms of aniline poisoning are quite different from those of the afectados. Laboratory tests proved that the oil was not harmful to animals. "All the animals thrived on the stuff," one researcher explained. "Their coats became glossier and they put on weight."

Dr Gaston Vettorazzi was chief toxicologist at the WHO at the time of the outbreak, but had since retired. He told us, in the most gracious way, that if even a bunch of journalists with no scientific expertise could see through all this, then it must indeed be obvious. In other words, he didn't believe that this had occurred through a series of administrative errors; he believed that the truth had been deliberately concealed by Spanish officialdom. As he said, the rapeseed explanation of the illness was "predetermined. That was the official line of the so-called Spanish science. You cannot force an investigator to follow a line. If this is done, science is dead."

For the various political and industrial concerns, there was substantial common interest in hiding the truth. For the multinational chemical companies, the revelation that a mass poisoning had occurred would have been scandalous and financially disastrous. At that stage, organo-chlorine (OC) pesticides were being phased out, to be replaced by organo-phosphates (OPs). The profits generated by the worldwide sales of OPs in the past 20 years have been vast. In those terms, suppressing the true cause of la colza was a commercial imperative. The Spanish administration had entirely congruent interests. With the attempted coup in parliament still fresh in the public mind, it was vital that ministers were seen to be in control. Democracy itself depended on the government being seen to deal capably with this national tragedy.

Moreover, at that time, Almeria represented an economic miracle for Spain, providing produce that went to all parts of Europe. Had it been frankly acknowledged that all those deaths had been caused by pesticides on tomatoes, the effect on the entire Spanish export trade would have been incalculable.

Nor was that the only economic repercussion. The news that such staple home-grown produce as tomatoes could be poisonous would have had a calamitous impact on Spain's other main generator of foreign income, the ever-growing tourist trade. On the other hand, spreading the fiction that the epidemic had been caused by cheap rapeseed oil sold in unlabelled containers at street markets to the Spanish working class in poorer areas of the country - that, of course, had no effect on tourism.

The consequences of the cover-up were appalling. Many died unnecessarily. Thousands more, children among them, were left to endure a lifetime of pain and physical impairment that perhaps could have been avoided if they had received the care and treatment they needed as early as possible. The Spanish colza is not just one of the great tragedies of the last century, it is also one of the great scandals.

Years later, in 1989, a similar mystery illness was first diagnosed in New Mexico. Victims, 29 of whom died, fell ill with pneumonia-like symptoms. Altogether, there were about 1,500 cases across the US. The symptoms appeared identical to those suffered by the afectados in Spain; yet no one in the US had had access to contaminated cooking oil.

It is virtually certain that this outbreak, too, was caused by OP pesticides. The scientific community - helpfully for their paymasters - did not conclude that; the cause of the illness was attributed to an innocuous amino acid supplement, L-Tryptophan, which had been taken without problem by millions of Americans throughout the 1980s. (Its sale is now banned in the US and Europe.) Just as with toxic oil syndrome, funding was available for scientists who wished to pursue the official line, but not for those who held different views. Nevertheless, no component of L-Tryptophan has ever been found that would account for the symptoms suffered by victims.

There have now been several issues about which there is a general perception that the truth is not being allowed to surface. These include, most obviously, the effects of OPs on farmers in Britain. Despite what appears to be a mounting toll of death and debilitating illness inflicted on the farming community, all official inquiries somehow fail to establish a link between pesticide exposure and the illness.

The WHO, to its shame, continues to refer to the Spanish epidemic as the "toxic oil syndrome". Every day around the world, students are no doubt being taught that "cooking oil" was the cause of the disaster. Two books on the cover-up have lately been published. One of these, Detras de la colza, is by Granero, Muro's right-hand man; the other, published in France, is Jacques Philipponneau's Relation de l'empoisonnement perpétré en Espagne et camouflé sous le nom de syndrome de l'huile toxique - but the worldwide deception continues, automatically recycled by a compliant media.

The enduring feature of the TOS saga is that it provided a blueprint for the international scientific community. If even a theory as palpably bogus as the "toxic oil" syndrome can be sustained internationally, then suppressing the truth must be remarkably straightforward. All it takes is a series of epidemiological reports, accredited by scientists of a similar persuasion, and then published in reputable scientific journals. There are, as Disraeli might have said, lies, damned lies and peer-reviewed scientific papers.

Given increasing privacy constraints, the media can never independently verify the data, and just have to report whatever they are told.

Moreover, we could discover the truth about the Spanish epidemic for two reasons: because the two-year trial ensured that otherwise unavailable information reached the public domain (and the authorities haven't made that mistake again); and because I was able, in 1990, to spend almost three months in Spain researching and filming the epidemic. A decade later, it is now inconceivable that journalistic investigations on such a scale would be supported. In future, without even the remote possibility of a bunch of journalists turning up years later to ask inconvenient questions, it will be even easier for international science to organise its cover-ups.

An internal German government memo was recently leaked to Der Spiegel. According to this, the monitoring of imported produce had revealed that there continued to be unsafe pesticide residues on fruit and vegetables from Spain. Some peppers were "highly contaminated" and the residues had "reached levels we can no longer tolerate". It was the last line of the memo that was most telling: "Under no circumstances should the general public be informed."