plague vs prey on someone's mind

plague

noun
  • A group of common grackles. 

  • A widespread affliction, calamity or destructive influx, especially when seen as divine retribution. 

  • The bubonic plague, the pestilent disease caused by the virulent bacterium Yersinia pestis. 

  • An epidemic or pandemic caused by any pestilence, but specifically by the above disease. 

  • A grave nuisance, whatever greatly irritates. 

verb
  • To harass, pester or annoy someone persistently or incessantly. 

  • To afflict with a disease or other calamity. 

prey on someone's mind

How often have the words plague and prey on someone's mind occurred in a corpus of books? (source: Google Ngram Viewer )