Researchers analysed two key brain chemicals
Scientists believe Alzheimer's and Parkinson's disease may have a common cause.
Researchers in the United States have found tboth diseases may be triggered by the same brain chemicals.
This could explain why some people with Alzheimer's show symptoms of Parkinson's disease and vice versa.
But it could also mean drugs developed for one disease could help to fight the other.
Professor Virginia Lee and colleagues at the University of Pennsylvania School of Medicine looked at two key brain chemicals.
They examined taut - a chemical that can stop the brain's neurons from working properly.
This is found naturally in the brain but is abnormally high in patients with Alzheimer's.
They also looked at alpha-synuclein - a chemical that regulates communication between neurons in the brain.
This is also found naturally in the brain but is thought to turn toxic and damage cells in patient's with Parkinson's.
The US researchers carried out laboratory tests on both chemicals.
They found the two can work together to create amyloid lesions - the cause of Alzheimer's and Parkinson's.
In both diseases, amyloid, which is a protein in the brain, clumps together and forms "plaques" killing brain cells.
They also found one chemical can help the other to grow.
"This fundamental relationship may explain why patients with one disease are more likely to exhibit signs of the other disease," said Professor Lee.
The researchers said the findings could improve treatment for patients with either disease.
"This newly uncovered interaction between these two proteins suggests that therapeutic agents created to directly or indirectly inhibit the formation of one form of amyloid lesion might be effective for treating other forms of amyloid lesions," said Dr Benoit Gaisson, one of those involved in the study.
"That is, a drug meant to keep Lewy bodies from forming to prevent Parkinson's disease might also help prevent tau tangles from forming in Alzheimer's disease."
The study is published in the journal Science.