VD vs syphilis

VD

noun
  • The design diving speed of an aircraft; the highest speed which the aircraft is planned to reach during flight testing. Always higher than VNE (the speed that must not be exceeded in service). 

syphilis

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

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