  The temporal and spatial development of Parkinson's disease has been
characterised as the progressive formation of {\alpha}-synuclein aggregations
through susceptible neuronal pathways. This article describes a new model for
this progression mechanism in which Parkinsonian damage moves over time through
the nervous system by the combined effect of the reaction kinetics of
pathogenesis and molecular diffusion. In the reaction-diffusion model, the
change from a healthy state to the disease state advances through the nervous
system as a wave front of Parkinsonian damage, marking its path by
accumulations of damaged {\alpha}-synuclein and neurotoxic levels of oxidative
species. Progression according to this model follows the most vulnerable routes
through the nervous system as described by Braak's staging theory and predicts
that damage will advance at differing speeds depending upon the level and
number of risk factors, in a manner that gives new insights into the variations
with which Parkinson's disease develops.
