Another study showed that injecting clumps of the protein alpha-synuclein into mice’s stomachs and intestines. Days later the mice had aggregates of alpha-synuclein in their intestines that had spread via the vagus nerve to the brain. Months later their brains had many alpha-synuclein aggregates in their brains that negatively affected their motor function. Additionally, gut bacteria have been shown to be significantly different between healthy people and individuals with Parkinson’s disease.