Do neurodegenerative diseases (eg. PD) start in the olfactory system - a recent paper
 

"Together, the literature reviewed herein support our argument that pathological changes following environmental insults might contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner."

Rey, N. L., Wesson, D. W. & Brundin, P. The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases. Neurobiol Dis 1-24 (2016). doi:10.1016/j.nbd.2016.12.013

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

(this is an "Open Access" article)