crab louse vs sexually transmitted disease

crab louse

noun
  • A parasitic insect, Pthirus pubis, that lives amongst the pubic hairs of humans and feeds on blood. 

sexually transmitted disease

noun
  • Any of various diseases that are usually contracted through sexual contact. 

How often have the words crab louse and sexually transmitted disease occurred in a corpus of books? (source: Google Ngram Viewer )