outbreak vs underground activity

outbreak

noun
  • An eruption; the sudden appearance of a rash, disease, etc. 

  • Synonym of breakout (“escape from prison”) 

  • An outburst or sudden eruption, especially of violence and mischief. 

  • A sudden increase. 

verb
  • To burst out or break forth. 

underground activity

How often have the words outbreak and underground activity occurred in a corpus of books? (source: Google Ngram Viewer )