Dementia, Parkinson^aEURTMs disease, multiple sclerosis, and motor neuron diseases cause significant disability and mortality worldwide. Although the etiology of these diseases is unknown, highly correlated disease prevalences would indicate the involvement of common etiologic factors. Here we used published epidemiological data in 195 countries worldwide to investigate the possible intercorrelations among the prevalences of these diseases. All analyses were carried out using nonparametric statistics on rank-transformed data to assure the robustness of the results. We found that all 6 pairwise correlations among the prevalences of the 4 diseases were very high (>.9, P<.001). A factor analysis (FA) yielded only a single component which comprised all 4 disease prevalences and explained 96.3% of the variance. These findings indicate common etiologic factor(s). Next, we quantified the contribution of 3 country-specific factors (population size, life expectancy, latitude) to the common grouping of prevalences by estimating the reduction in total FA variance explained when the effect of these factors was eliminated by using the prevalence residuals from a linear regression where theses factor were covariates. FA of these residuals yielded again only a single component comprising all 4 diseases which explained 71.5% of the variance, indicating that the combined contribution of population size, life expectancy and latitude accounted for 96.3%^a^'71.5%=24.8% of the FA variance explained. The fact that the 3 country-specific factors above accounted for only 24.8% of the FA variance explained by the original (ranked) disease prevalences, in the presence still of a single grouping factor, strongly indicates the operation of other unknown factors jointly contributing to the pathogenesis of the 4 diseases. We discuss various possible factors involved, with an emphasis on biologic pathogens (viruses, bacteria) which have been implicated in the pathogenesis of these diseases in previous studies.