59. However, these very social changes, which have created and increased the need of cooperation between the clergy and laity to which We have just referred, have themselves brought along in their wake new and most serious problems and dangers. As an after-effect of the upheaval caused by the Great War and of its political and social consequences, false ideas and unhealthy sentiments have, like a contagious disease, so taken possession of the popular mind that We have grave fears that even some among the best of our laity and of the clergy, seduced by the false appearance of truth which some of these doctrines possess, have not been altogether immune from error.