“It is a vicious cycle,” says Justin Rustenhoven at the University of Auckland in New Zealand. This may also help explain why Parkinson’s disease is a progressive condition that worsens over time, he says.

Parkinson’s disease, which affects more than 10 million people worldwide, is characterised by a loss of neurons in brain regions that control movement. This causes symptoms such as tremor and muscle stiffness. The condition is thought to be caused by alpha-synuclein, which occurs naturally in the brain, misfolding and forming clumps that damage neurons.