crab louse vs syphilis

crab louse

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

syphilis

noun
  • A disease spread via sexual activity, caused by the bacterium Treponema pallidum. 

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