The cell-to-cell transmission of the major pathogenic proteins of Parkinson's disease and Alzheimer's disease is reminiscent of the prion protein, which is defined as a proteinaceous infectious particle that causes human and animal transmissible spongiform encephalopathies. The possibility has raised that the pathogenic proteins of Parkinson's and Alzheimer's disease are infectious, i.e. that they can transmit disease from human to human. In this review, we address this question by comparing the similarities and differences between Alzheimer's disease/Parkinson's disease pathological proteins and prions and by discussing the possible consequences for disease transmission risk.