In 1998, Michael J Fox, the elfin star of the US sitcom, Spin City, lost his Peter Pan image in one startling announcement: he had, he told us, been diagnosed with Parkinson's disease seven years previously. To most of us this didn't seem possible. Fox - who looked about 16 years old - was in his 30s when diagnosed. How could he have a disease normally associated with the elderly? It appeared to be genetic bad luck on a grand scale.

It may, it has now emerged, have been more than that. Fox's first big TV break came with the 1970s Canadian TV sitcom Leo and Me. And it emerged last week that three other members of this TV crew have also been diagnosed with Parkinson's. Scientists are arguing over whether this could be classed as a "cluster" of Parkinson's cases - and if so, what could have caused it?

For more than a century scientists have been arguing about what causes this debilitating, degenerative brain disease. Some believe that the condition is a result of still-to-be-discovered genetic factors. Others maintain that most cases of Parkinson's are caused by the world around us: exposure, perhaps, to some kind of viral infection, or to environmental pollutants. The Leo and Me news merely highlights how little we know about a disease that hits about 1% of people over the age of 50. Can you "catch" Parkinson's? Or is it all down to fate?

The disease is certainly not new. Before the English doctor James Parkinson described its symptoms in 1817, people called it the "shaking palsy". It is, says Dr JW Langston, scientific director and chief executive of the Parkinson's Institute in California, "a progressive neurodegenerative disorder characterised by stiffness of muscles, slowing down of movement and a tremor".

The slowness (known as brady- kinesia) and shaking that we all recognise as a sign of Parkinson's worsen as the condition progresses (usually over a few years). This is because certain cells in your brain are simply fading away. "These cells - the pigmented neurons in the substantia nigra of the brain - produce dopamine, a chemical messenger responsible for transmitting signals within the brain," Langston explains. "If there is not enough dopamine, critical nerve cells in the brain, or neurons, start to fire out of control."

Eventually, you find walking, talking, or doing other simple tasks extremely difficult. But in the beginning it can be difficult to diagnose. "The average time from the first symptom appearing to diagnosis is two years," says Langston. The Michael J Fox Foundation, which supports research into Parkinson's, estimates that while "at least one million people in the United States ... have Parkinson's, many of them, perhaps half, are thought to be undiagnosed".

One thing, though, is beyond debate: Fox was startlingly unlucky to develop it in his 30s. His "young-onset" Parkinson's, which affects those under the age of 40, accounts for only 3-4% of cases. Doctors understand more about young-onset than they do older-onset Parkinson's. There is, Langston says, "strong evidence that this kind of Parkinson's is highly genetic". However: "When it comes to most cases of typical older-onset Parkinson's, genetics don't look likely as an explanation."

In 1999 Langston's team published a study in the Journal of the American Medical Association in which they compared instances of the condition in twins with exactly the same genetic make-up and fraternal twins. The rate of Parkinson's in both types of twin was virtually the same. They concluded that there was little evidence that the cause of the illness in older people (ie those over 50) could be genetic. Consequently, says Langston, "most people would currently say that the environment plays a greater role in Parkinson's than genetics".

Dr Paul Maestrone, director of scientific and medical affairs at the American Parkinson's Disease Association, emphasises that "no definitive data exists" to fully explain the causes of Parkinson's. And the Michael J Fox Foundation is equally cautious, saying on its website that "most scientists believe a combination of genetic and environmental factors" are to blame.

But what, exactly, in our environment could be causing it? "That is the 64 million dollar question," Langston says, "and it is why clusters of the disease are so interesting: they could point to an environmental cause - some common exposure." The problem is that clusters are "incredibly difficult to investigate - you would have to do daunting research to get an answer". It is, for instance, extremely difficult (and expensive) to differentiate between a cluster and a coincidence. And it is even more difficult to establish, from that, what could have caused the outbreak.

"You might trace all the people in a cluster to one apartment block," Langston explains, "then discover that it was torn down in 1982 and made into a mall." Even if the building still exists, toxins once present may have dissipated. Remaining evidence may be scarce, or patchy. "We once put together a proposal to set up a team which would investigate all suspected clusters of Parkinson's across the US," Langston says. "We were going to be called the 'Cluster Busters'. But we didn't find funding."

This difficulty could explain why other suspected Parkinson's clusters have remained enigmatic. In one case, at the University of Pittsburgh, four people who worked in the same department were diagnosed with the disease between 1974 and 1991. This rate was 30% higher than the general population. The department of neurology convened a "cluster committee" of experts to investigate but they could find no evidence of an environmental cause.

There are, however, some environmental culprits that show up in study after study, says Maestrone. Investigations into incidences of Parkinson's in agricultural communities, for instance, have repeatedly linked high exposure to certain pesticides, herbicides and fertilisers with the disease (one Canadian study of farming communities showed that those who drank well water were dramatically more likely to develop the disease. The water table, they concluded, had been contaminated by pesticides and herbicides).

Other investigations into the disease in mining communities have shown certain metals to be guilty parties - aluminium and manganese are the main villains. But then again, other investigations, going back over five generations of a family with an astonishingly high rate of Parkinson's led scientists to identify two genes that cause a particular kind of the disease. However, since these genes are extremely rare, these studies by no means account for most cases of older-onset Parkinson's.

So what about Fox and the suspected Leo and Me cluster? Did they all breathe in some environmental toxin in the studio? Or did they perhaps pass the disease to each other? One theory, that has been around for years, is that Parkinson's could be caused - or perhaps triggered - by a viral infection, maybe even influenza.

Dr Donald Calne, of the University of British Columbia Hospital, is treating two of the cast of Leo and Me. He told the Chicago Sun-Times that studies have found there to be an increased risk of clusters among certain workers who operate closely together - principally teachers, medical workers, loggers and miners. They could, the theory goes, be exposed to more viruses than most people, and this might cause them to develop the disease.

"You can find studies that people who spend time with each other are more susceptible to Parkinson's disease and you can find the reverse," Dr Abe Lieberman, medical director for the National Parkinson Foundation in Miami, told the Sun-Times. "If Michael J Fox's wife developed Parkinson's disease, I would pay more attention to the cluster theory."

So right now, as Langston puts it, "there is no smoking gun." The outlook, however, is not entirely bleak: Langston believes that there is certainly "a good chance of finding a cause of Parkinson's in the next 10 years". The cause, of course, is not the cure. But if we know who's pulling the trigger we might, eventually, be able to dodge the bullets.