The cause of Parkinson's Disease is unknown. To date, many experts have suggested that it may have a genetic origin; see http://www.webmd.com/parkinsons-disease/parkinsons-causes. Other experts point to toxins that may selectively destroy the dopaminergic neurons, thereby causing Parkinson's Disease. Toxins that have allegedly been linked to Parkinson's include manganese, carbon monoxide, carbon disulfide, and some pesticides.
Now, researchers (using an animal model) have found evidence that the cause may be Helicobacter pylori, a bacterium that lives in the stomachs of about half the people in the world and causes ulcers and stomach cancer. [For those with long memories, you may recall that ulcers use to be associated with stress and/or stomach acid until researchers found the cause, although they were subjected to years of abuse for their "folly". Ultimately the two researchers received the Novel Prize in medicine for their discovery. See http://health.dailynewscentral.com/content/view/0001725/37/.]
Middle-aged mice infected with the ulcer-causing bacterium developed abnormal movement patterns over several months of infection, the researchers have noted; yet, young mice infected with the bacterium did not show any signs of movement problems. The researchers found that Helicobacter-infected mice make less dopamine in parts of the brain that control movement, possibly indicating that dopamine-making cells are dying just as they do in Parkinson's Disease patients. The bacteria did not have to be alive to cause the problem. Feeding mice killed H. pylori produced the same effect, suggesting that some biochemical component of the bacterium is responsible. A candidate for the disease-causing molecule is modified cholesterol. Helicobacter is unable make its own cholesterol, so it steals cholesterol from its host and then sticks a sugar molecule on it. The structure of the modified cholesterol resembles a toxin from a tropical cycad; people in Guam who have eaten the plant's seeds have developed a disease called ALS-parkinsonism dementia complex. The researchers are now seeking to determine if the modified cholesterol alone can lead to Parkinson-like symptoms in mice or if some other factor from the bacterium is also needed.
As with all things biological, there is a problem. Even if researchers can show that H. pylori can cause or contribute to Parkinson's disease, it is not clear whether getting rid of the organism would be a good thing. Although the bacterium causes ulcers and stomach cancer, it also helps protect against allergies, asthma, esophageal cancer, and other acid reflux diseases. Where would be the balance? No one at present knows. As they say, stay tuned.
The study was reported this month at the 111th annual meeting of the American Society for Microbiology: http://gm.asm.org/index.php/component/content/article/48-newsroom/211-ulcer-bacteria-may-contribute-to-development-of-parkinsons-disease.
This is only anecdotal evidence and proof of nothing, but both people I have known who suffer from Parkinson's Disease also had intense gastrointestinal issues.