Maybe I can have a crack at that one. That's a fantastic question. That's a question that's at the centre of a lot of the research that's going on.
Clearly, as you've heard today, there are almost certainly environmental factors, maybe pesticides, maybe other factors, and hopefully we'll have some answers about those, but you're absolutely right: people living in the same house who are doing the same work, one gets Parkinson's and the next person doesn't. It's almost certainly a combination of the environment and genetic susceptibility. These are the two big factors that come into play about exactly who develops Parkinson's.
You may have someone who has a certain combination of genes whereby no matter how much pesticide he may see in his lifetime, he would never develop Parkinson's, whereas someone else may be extremely susceptible, even though they are very mildly exposed to it.
This also plays into the question of personalized medicine. Probably certain genetic and environmental factors will make people more responsive to certain treatments. Now we treat almost everyone with Parkinson's in a similar way, whereas we think that in the future, with personalized medicine, which is a field that's just in its infancy, if we can say a person is much more likely to respond to one medication than another, or someone might be more responsive to music than to tai chi.... There is another study showing that patients with Parkinson's who do tai chi have fewer falls when they practise tai chi. This is the kind of thing where we'll see a big shift in the future.